Not exlusive to this forum but I find that if the 'keep me logged in' box isn't ticked then I can find myself logged out when trying to post a reply.. can be pretty scary if I've spent a time typing a long post!
Is there an automatic logout time of 20 mins or so or a certain amount of innactivity or is there something else going on?
As I say, it happens on other forums too if I don't have 'keep me logged in' ticked.