Kubuntu 12.10 Desktop keeps locking

[Bumpalot]

Kubuntu 12.10 Desktop locks after using for about 10 mins. Absolutely nothing can be activated, except Ctrl+Alt+F1, then Ctrl+Alt+F7 to return to Desktop. when all is OK for a short while then repeats again.
Any ideas what is causing this annoying problem?

[Snowhog]

Have you run the memory test from the the Grub menu? Eliminate the possibility that you might have a bad memory chip.

[Bumpalot]

Have tested Grum - memory - No problems.
What now

[SteveRiley]

Next time it happens, switch to TTY1 and then back to X (TTY7) like you're able to do. Then post a copy of /var/log/syslog to pastebin.ubuntu.com and reply here with the URL. Perhaps the log will present some clues.

[Bumpalot]

http://pastebin.ubuntu.com/5727115/

Since I posted the above. my system is CONSTATLY crashing - have to keep switching to TTY1 and then back to X (TTY7), runs fine for 15 - 45 mins then repeats. It's hard to figure out WHY!