Java Programming

Hi,
I got a rather annoying problem with users being logged out from time to time, without an apparent reason that Im in need of help for. In the system a user can be logged out, no matter what time on the clock, in pretty much all areas, its not decided by how long you have been logged in, and its not bound to a specific user.
The only thing Ive noticed is that the more users the system have gotten, the bigger and more apparent the problem has become. When the system had a smaller amount of users, it was much easier to just announce it as a random occurence, and they were refered to logon again a bit later, and it works. But now, it concerns more and more users, and its not acceptable any longer to refer them just to logon again. It shouldn't occur, but it does.

I know a user actually is being logged out by session death, because the system logs if the user chooses to logout. Also the system logs when a user is logging in. And when a user is logging in twice within a minute or two, its quite apparent it has happend(besides from the user saying it happend).

Ive searched the web but its easier said than done. And Ive gone through a specific code part where the problem is said to occur a bit more often than other places, without any luck.

So Im wondering if there is any reason to why a user's session can die so randomly? Is it even possible, that a session can just die for no apparent reason? Could this be a network problem between the servers? Or is it just a subtle bug lurking someplace in the code?