I never had it happen on my computer, which runs IE 7, but coolduff's runs IE8, and it happened 3 times in a row- auto log out between competitions sign up. Id sign up for wit, then go to sign up for will, and I would be logged out. Less time had passed then time I stayed on forums.. I was in a game where you could post every 5 minutes for a number guess, and watched TV for 10 minutes at a time, came back and posted, no log out. Competition sign ups dont take 10 minutes, but logged out 3 times.
Not sure if its an IE8 issue or what, I dont think coolduff competed enough to ask him if it ever happened for him.
For the moment I'm going to assume it's a cookie issue. Next time you're on that computer, clear out the cookies and cache before logging in. If it happens again and you're sure you've done a cache clear, let me know and I'll see if I can figure anything out.