After delaying the upgrade from Snow Leopard to Mountain Lion (I tried a clean install and this was working fine for me) I decided to upgrade.
Worked fine for a day or so and now I have the maching logging me out completely after a couple of minutes on three accounts tested (including the guest account).
I have worked through most of the solutions suggested
1. Repairing permissions using disk utilityand resetpassword (repairing ACL permissions didn't seem to work or just took forever!)
2. Booting in safe mode (which appreared to be stable) and removing potentially offending automatic login items
I do not use dual monitors nor is there a dual graphics output ( no built in graphics on Core 2 duo that I'm aware of) so i suspect this is potentially a software issue.
I also cannot use my USB audio driver as I cannot see this at all in Audio MIDI drivers or in the sound option.
Very disappointed with this and I plan to revert to Snow Leopard until this is resolved (praying that time machine will let me do this!).
intel core 2 duo iMac 24 2009