Do you ever have to re-login after you experience weirdness? Like, do you ever get prompted to reenter your username/password when you wouldn't normally expect it? Sometimes (at least my computers) IE seems to randomly stop loading some cookies, and it makes similar behavior happen here and elsewhere for me. Weirdness can also happen if you access the site from more than one computer.

I can't think of anything else that would cause that to happen. Next time it does, try scrolling to the bottom of the main forums page and seeing if it shows a login box. If so, it's probably IE oddness.

I'll think about it some more and see if I can come up with anything.