session keeps expiring

[eluser]keevitaja[/eluser]
i'm having problem with ci session expiring. maybe after 5 minutes or so. anyone has an idea why it is doing that?

Code:

$config['sess_cookie_name']  = 'ci_session';
$config['sess_expiration']  = 0;
$config['sess_expire_on_close'] = FALSE;
$config['sess_encrypt_cookie'] = TRUE;
$config['sess_use_database'] = TRUE;
$config['sess_table_name']  = 'ci_sessions';
$config['sess_match_ip']  = TRUE;
$config['sess_match_useragent'] = FALSE;
$config['sess_time_to_update'] = 300;