I think I experienced this (and I think this was also the responsable for my video problems)

First, I would be 24 hours Prime95 stable, but then I would go watch a movie and all you know after 10 secs the PC freezes.
Then I would also leave the PC in idle, leave for 30 mins come back, move the mouse and 5 secs, restart.

I updated my BIOS and RMAed the board and problems were gone. Since then I still havent overclock my system (wating for water cooling), but once I do I hope LLC doesnt cause any problems.