I have noticed that since the shoutbox was added, random logouts happen. I might have found the issue and a solution for it. Always when it happens for me, I have two forum windows open on my browser. This is because I leave one open for the shoutbox and the other one for posting. What that can potentionally happen is that because the chat refreshes constantly, the session gets lost because of the new window being open.
I had this problem the other day constantly, I was trying to login to the admin area and kept getting signed out. I closed the second window and did everything in one window which solved it.
There you have it. I am not sure if all of this is incorrect until more testing has been done.