I think my problem may be related:
also seems to happen more often when mozilla is running.
have any of you tried to access your machine over the network after a freeze? if you have a 2nd computer with an ssh client you might find that you can still log in remotely. and if you run the program "top" (or simply ps afux) you'll probably find that X is using 100% of the CPU.
you can run "killall -9 X" which will hose the console, but then you can restart X (or if you use runlevel 5 it will start up again automatically).