Session logout Problem -- db sessions

[eluser]Cro_Crx[/eluser]
There's a lot of factors here, it would be good to get more information (specifics)...

Have you tested multiple browsers ?
What version of CI are you running ?
Are your database tables using transactions ?
Have you inspected the session information when these events occur ?
Are you storing your session cookies in the database or in files ? (sounds like database from above)
How much information are you storing in your cookies ? (File cookies only have 4k limits which sounds like a lot but when you have $config[‘sess_encrypt_cookie’]= TRUE; then it's not very much at all!