I'm not sure if this is an Ubuntu question, or a Linux question in general, or perhaps an Internet question.

Anyway, my internet has been working fine until recently (last week or so). For most sites, Firefox will load the page quickly. However, every once in a while, but frequently enough to annoying, it will say "loading" for 10 seconds, then direct me to OpenDNS, which says the page cannot load. When I try it again, I get the same problem. Other sites are fine.

Then, when I quit Firefox and restart it, that site will work fine, but soon enough, another site stops loading properly.

For example, Google will work fine for awhile, then I'll get the error, but after restarting, Google will be fine again, but now Wikipedia, which loaded before, gets the error. And the cycle continues on yet another website...

I haven't changed any Internet or network settings recently. I'm on Ubuntu 10.04.

Well, it looks as if you are having problems with DNS. given that it is going to openDNS, presumably they are set up as your DNS provider (or one of your DNS providers).

It would be as well to ensure that ipv6 is turned off.

Otherwise, you'd be tempted to suggest that the problem is at OpenDNS, but, as far as I know, they are fairly reliable. You could, I suppose have some form of content filtering turned off at OpenDNS, but why that would object to Google or Wikipedia unless they have some form of extreme sandboxing, I don't know.

Can you post /etc/resolv.conf to check that there is not something strange hiding in there? And, is anything dynamically overwriting resolv.conf (networkmanager, PPPoE) which could lead to different settings after reboots?

Turning off ipv6 seems to have solved it. Thank you.