OK, I've actually discovered what the issue was when I was setting the forum to cache the user session data via an opcache. It's due to us running php as fcgid. I think I've fixed this, as there were several things set allowing multiple php processes to be started, causing users to jump between them (1st issue if you hit the process which didn't have your session in the cache), and having a lifetime on the process of 2 hours, so the maximum time the cache would be available was also two hours, as each php process gets it's own cache.
I've made some changes which are now active, and can see the cache is now attached to a single php process
# ps -ef | grep 25157
z22se 25157 25014 3 10:15 ? 00:00:22 /usr/bin/php

Will keep an eye on it over the next couple of days :thumbup: