[eluser]pickupman[/eluser]
Many have had better luck changing in application/config/config.php the 'sess_cookie_name' from ci_session to cisession. (remove underscore). Also, are you using AJAX in your application. If so, AJAX can cause a race condition that will mess up a session as well when sending two simultaneous AJAX requests. The one AJAX request may cause the session id to get rotated from garage collection, and the 2nd request will fail because the cookie will no longer be valid.