I'm using CiviCRM 4.6.5, Joomla 2.5.28 (yes, I know...), with 70k contacts and 25 groups (one of which is a smart group).
Since the upgrade to 4.5.6, contacts started to display the wrong groups (actually several times the same group, instead of several different groups). If you check the groups the contact should be in, the contact is there. But the group doesn't show on the contact record. Cleaning the Caches (through the backend) seems to fix the problem, and everything is fine and dandy for a while, with the groups displaying correctly in the contact records again. But the fix is only temporary - the next day, the problem is back again.
Can anyone please suggest where to start looking for the cause?