Users lose CI session randomly and have to delete local cookie prior to logging in again

[eluser]echadwickb[/eluser]
I am having a time with the sessions class in CI. Users get randomly logged out in one of my apps, and are unable to log back in until they remove the local cookie from their machine.

Server Environment:

IBM i V6R1
DB2 (using db2c driver)
PHP via CGI in Apache (or whatever the heck Zend Server is doing behind the scenes)
CI 1.7.2 in production (2.0 in development)

Clients: Chrome 8/IE 8/Firefox 3.x

Either sess_destroy() is not running or setcookie() within sess_destroy() is failing to invalidate the local cookie. I have a feeling that I'll be coming up with a solution as soon as I post this entry, but troubleshooting tips are much appreciated.