You may have to update the session.save_path value as well. Sessions are stored as files in a folder specified by that path. If another shared site on that server goes through and clears out sessions that are more than 24 minutes old, it may remove yours as well. A unique save_path could be the solution.
It's always best to use a session.save_path in the directory owned by your user outside of the webroot that has full read/write/execute permissions (777).
Finally, don't forget that you need to set the session.cookie_lifetime to be greater than, or at least equal to, your session.gc_maxlifetime directive.
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