Well I have tried that site 3 times. Once it confirmed that checkip.dyndns.com was down for everyone and the other two times it said it was just down for me. I guess all it says is what we all suspected - it's an intermittent fault...

I recommend that you contact DynDNS Support about these issues.

By the way, for ddclient users, it may be an option (depending on your router) to query the IP directly from the router instead of doing remote lookups against checkip.dyndns.org. That way, you can also safely reduce the interval between checks to one minute or less.

My testing indicates that the problem is intermittent with all combinations of checkip-ams.dyndns.com and checkip-ewr.dyndns.com, ports 80 and 8245. http://downforeveryoneorjustme.com/ indicates that the sites are up.
The internet connection appears to be fine otherwise. So I'm led to believe that there's a routing problem somewhere between me and the checkip hosts. I'm coming from a Qwest location. Here's a tracert fwiw:
[root@lanyard ~]# tracert checkip-ewr.dyndns.com
traceroute to checkip-ewr.dyndns.com (204.13.248.70), 30 hops max, 40 byte packets
1 pinkerton.lan (192.168.5.2) 1.146 ms 1.094 ms 1.051 ms
2 192.168.0.1 (192.168.0.1) 4.534 ms 5.097 ms 7.961 ms
3 phnx-dsl-gw12-204.phnx.qwest.net (67.40.227.204) 49.797 ms 53.959 ms 54.397 ms
4 phnx-agw1.inet.qwest.net (216.160.199.89) 56.899 ms 57.439 ms 58.177 ms
5 lap-brdr-03.inet.qwest.net (67.14.22.74) 67.312 ms * *
6 * if-2-5.icore1.EQL-LosAngeles.as6453.net (206.82.129.33) 75.262 ms *
7 * if-4-28.tcore2.LVW-LosAngeles.as6453.net (216.6.84.53) 70.364 ms *
8 * if-2-2.tcore1.LVW-LosAngeles.as6453.net (66.110.59.1) 58.624 ms 59.146 ms
9 if-10-0-0-712.mcore3.LAA-LosAngeles.as6453.net (66.110.59.10) 60.717 ms 58.485 ms 59.570 ms
10 if-4-0.mcore3.NJY-Newark.as6453.net (216.6.84.2) 521.730 ms 512.645 ms 509.199 ms
11 if-14-0-0-726.core2.NTO-NewYork.as6453.net (216.6.57.58) 504.513 ms 477.816 ms 474.246 ms
12 Vlan1260.icore1.NTO-NewYork.as6453.net (209.58.26.66) 472.956 ms * *
13 Vlan572.icore1.NTO-NewYork.as6453.net (209.58.26.54) 124.051 ms 124.082 ms 123.678 ms
14 checkip-ewr.dyndns.com (204.13.248.70) 121.651 ms 125.026 ms 126.509 ms
[root@lanyard ~]#

I'm no networking guru. What's next to diagnose the problem?

I've had this since the 12th, every second hour or every hour.

If I do a traceroute I get different results, but most of them end in the same way:
# traceroute checkip.dyndns.org
traceroute to checkip.dyndns.org (204.13.248.70), 30 hops max, 60 byte packets
1 c-99-99-99-99.cust.bredband2.com (99.99.99.99) 5.542 ms 5.375 ms 5.207 ms
2 xe-1-1-0.stk30.ip4.tinet.net (213.200.72.5) 31.045 ms 31.028 ms 30.991 ms
3 xe-11-2-0.ams20.ip4.tinet.net (89.149.186.106) 25.740 ms 25.794 ms 25.760 ms
4 Vlan588.icore1.AD1-Amsterdam.as6453.net (195.219.150.53) 29.490 ms 29.494 ms 29.461 ms
5 if-5-0-0-1135.core2.AD1-Amsterdam.as6453.net (80.231.81.17) 25.902 ms 25.895 ms if-1-0-0-82.core2.AD1-Amsterdam.as6453.net (80.231.81.25) 26.138 ms
6 if-15-0-0.core3.NTO-NewYork.as6453.net (80.231.81.46) 107.266 ms 107.314 ms 107.531 ms
7 (63.243.186.37) 109.644 ms 108.629 ms 109.178 ms
8 Vlan572.icore1.NTO-NewYork.as6453.net (209.58.26.54) 109.162 ms !X 109.105 ms !X 108.924 ms !X

or

4 Vlan588.icore1.AD1-Amsterdam.as6453.net (195.219.150.53) 25.989 ms 25.958 ms 25.921 ms
5 Vlan576.icore1.AD1-Amsterdam.as6453.net (195.219.150.38) 26.321 ms !X 26.312 ms !X 26.277 ms !X

I.e all time value in the last line are marked !X which means "communication administratively prohibited". What's that?

On a subject which might also be related. If I try to open checkip.dyndns.org in my browser, F-Secure throws a fit and states:

http://checkip.dyndns.org/
This site is: Harmful
You should avoid visiting this web site F-Secure has found evidence of harmful behaviour on this web site. To the best of our knowledge, it is not safe to visit this web site.

Is is possible for some network operators to blacklist harmful sites?

Yup, even when that assessment is wrong

Can I request that you flag that up to F-Secure as an False Positive and raise a ticket with DynDNS support too.

They obviously don't want support emails at F-Secure... awfully hard to find an entry-point for information input.

Anyway, some staff at DynDNS.com could visit http://browsingprotection.f-secure.com/swp/
enter checkip.dyndns.org and click on Site Owner Feedback

However this is a user to user forum, so DynDNS staff are probably unaware, hence why I suggested you open a ticket

YES - IT IS NOT JUST MY ISP. Why would anyone suspect that EVERYBODY'S ISP is faulty when we have disparate reports of the SAME ISSUE from around the world?

From http://downforeveryoneorjustme.com/checkip.dyndns.org:

It's not just you! http://checkip.dyndns.org looks down from here.

A sample from my spammed message log:

Oct 14 08:56:17 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 09:47:03 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 11:01:37 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 13:04:03 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 13:46:48 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 16:16:06 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 17:12:27 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 14 17:35:24 asusb202 ddclient[14614]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 00:07:55 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 00:25:40 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 02:16:11 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 02:55:09 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 04:13:35 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 04:30:20 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 06:22:42 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 07:23:26 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 07:48:36 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 15 08:00:33 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 10:10:52 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 11:02:56 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 11:56:41 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 13:36:28 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 14:15:27 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 15:14:35 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 16 17:46:44 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 00:32:31 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 02:51:54 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 04:16:23 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 04:32:41 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 05:18:38 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 06:31:54 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 07:20:13 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 08:38:11 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 12:05:38 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 12:31:35 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 12:50:32 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds
Oct 17 15:16:47 asusb202 ddclient[26368]: WARNING: TIMEOUT: checkip.dyndns.org after 120 seconds

I've changed my '/etc/hosts' to resolve to 204.13.248.70 and it still keeps shitting the bed.
The original problem occurred with 91.198.22.70 after about 5 years of no problems of this sort with the same IP, so I'd say, yeah, the problem is on DYNDNS's side.

Great, so now you've confirmed that it's not just you (which was a worthwhile test since obviously not everybody is seeing the problem, including myself) open a ticket with DynDNS.

This, as it's title suggests, is a user to user forum. Posts here are highly unlikely to ever attract the attention of DynDNS staff.