This used to happen to me, but hasn't in a while.
Can you keep can eye on top, does CPU usage go to 100% when this happens? IIRC it was an issue with the "intel" graphics driver and some IGPs. Those affected would see their CPU usage go to 100%, and the lock up would happen.
The good news is I have been running jessie since he became testing, and that problem has more or less disappeared (happens very rarely, and sometimes can even wait it out).
Last edited by goumba; 10-15-2013 at 07:19 AM.