Te busque en google, me salio este articulo:
http://www.scyfox-site.com/2009/11/2...-sesiones-php/
Aqui hay otro con tu problema y al final resolvio que era un problema de cookies de su navegador:
http://www.forosdelweb.com/f18/probl...er-7-a-449636/
Ya me diras si te sirvio......
EDITO: aqui encontre algo mas de
http://php.filearena.net/manual/kr/r...sion.php#62450 Internet explorer users beware.
When using session_start() to begin a session this session will remain open until the page has finished loading or it is explicitly terminated.
You can lose the session however if the the page contains a reference to <img src=""> with name and id references (which may be used if the image is referencing a dynamic image, called by javascript) This seems to casue IE6 to refresh the page session id and hence loose the session.
This took hours for me to diagnose when users were getting unexpectedly logged out of my site due to this "" in the img src.