No idea what the issue is here, but I've had a similar problem. Firstly, is it really a hard lock? I thought mine had, too, including a keyboard freeze, but I was still able to ssh into the box over ethernet. This could provide valuable diagnostic information, as can the system logs: have you checked them?
I don't know how much you know, but do you know about using ctrl-alt-fn1 to fn6 virtual terminals? Can these get you out of the lock up? Also, when I had the same issue, switching from the X terminal to the virtual consoles caused screen corruption, but not a lock up. using ctrl-alt-f7 took me back to an uncorupted x display.
hope that gives you some leads.