Session expires randomly

Hi, I have this problem , When I start a Session everything seems to be ok but sometimes with no reason the session vanishes. I'm using PHP 4.3.4 as a Apache module. Apache version is 1.3 under Suse Linux 8.2 All settings are default , I mean session_cache_expire is 180 min. I understand that this setting should make sessions last for at least 3 hours but in my case it seems not to be true since the real duration varies from 20 minutes to an hour I use session_start() and then on any routine I verify the existence of certain session variables. Is the server expiring the session automatically for some reason? Thanks Mauricio