session mysteriously killed


we have an web based intranet system, users started reporting that they kept getting logged out, and the log outs were happening randomly.

There is no coded any were that refers to sessions other than the login/logout and header file.

Ive increased the session.cookie_lifetime to something stupid like 6 days, it was initially zero,, I might set it back to this, I also increased the session.gc_maxlifetime to a huge value, but still the problem persists.

I have a bit of code in the header (where we check if the user has valid session credentials) logging messages when someone is logged out and from what file they were accessing with they got logged out, there is no discernible pattern, Im stumped,

Does anyone have any idea how the session variables can be disappearing?