Session Problem

[BuLB]

I have a site which is running for few months already  :D .users login and chat with fellow members and read forums,blogs etch  ;) .its a community website.session works fine as what i want.they can idle and talk to the shoutbox when they back after a couple of hours  8).

but just lately i have a problem with it  :'(.sessions keep on expiring on a given time even if you are not idle.  :-[ it s kind of session expire or session destroy thus users are logout and need to login again but i didnt do anything on the script that i program.anyone of you here knows what to do and what to adjust? i need it to be back to the old style they will not be logout until they logout or close the browser  ???