Users automatically logging out

Hello, I've recently created a forum, and I have a problem that I can't fix. I've been on other forums hosted by this same service which did not have this problem. My members keep logging out automatically while they're creating posts of great length or even mild length. I've gone through everything I can think of to find the source of why this is happening, but I can't find anything. Could any of you possibly advise me as to how I can fix this problem? It's discouraged a few members quite a bit to lose a twelve paragraph post because they were logged out while creating the post.

Can try the following

Clean their Cache
Try FireFox
Login Problems