Q: Mountain Lion keeps logging user out...

I had the exact same thing. Right after applying the latest software update to enable Power Nap. I would log in, then it would restart all these previously opened apps. Once it finished opening everything, it would log out right away. I got like 1 minute each time before it would log out. I just rebooted the machine, instead of logging in again...that seems to have fixed it.

Hmmm. Have you tried reinstalling Mountain Lion?

We could be looking at a corrupt ML I dont know at this stage since no sufficient information has been posted yet. Reinstall looks to be a solid step for now

Right, can you try to repair disk pemissions?also is this happening in safe mode?

I would assume that something in that user account is not compatible with Mountain Lion which is causing it to crash. You can find out more in the logs by going to Applications/Utilities/Console

http://support.apple.com/kb/ht1528

If you are unable to create a new user account because its crashing. Enable root user following that link above and see if its happening there

yes because safe only loads apple kernels, basically it does not load any 3rd party daemons etc. That is to isolate whether this is caused by Apple software and hardware or a 3rd party app

so if safe mode then that means that a 3rd party(most likely) is causing that crash. You can check it in the logs via the console in the utilities folder

but if it doesn't then that simply tells us that something is wrong with that user account. Creating a new user account would dive us deeper into finding what's the culprit. Whether its a user account issue or a system wide issue