When a session is used by a 'main' script which causes a new browser window popup that also needs the session information (the session is called twice independently by the same user in one mouse-click) the session will be destroyed most of the time and sometimes PHP crashes. This problem occurs too when a shockwave object is loaded that also calls for the session information.
It is possible to prevent this autodestruct by putting an empty echo (e.g.: echo "";) after the second time the session is called (with: session_start();). 

I'm using MySQL for session storage, maybe this causes the  problem.