I don't think it is/was a graphics hardware issue for me. I never had the screen corrupted, visually, as you describe - it just froze.
However, it interests me what keith181 says above, as that has been my experience these past 2 weeks as well: I have done nothing in the last 14 days (besides installing 10.8.1) and my machine has only crashed twice in that time - the first time was before I installed 10.8.1, so it has only frozen once since then. Before that, it was freezing a couple of times a day. It is also fast and zippy and not sluggish (nor was it, before).
But I do have an interesting thing to report with my My Book backup hard drive which leads me to wonder whether our problems could be due to non-Apple external HDs?? And/or Time Machine functioning with these??
I bought the My Book in May, from the Apple Store - so it was not old. Yesterday, I got an error message telling me that my machine had not been backed up for 10 days(!!) (bit late notice - 10 days is quite a long time!!).
When I checked, the My Book HD light was flashing on and off. My iMac was unable to see the HD to back up to it and it wasn't on the desktop either. I disconnected the Firewire connection, then reconnected it - nope. I disconnected the power supply...
...and the My Book wouldn't power on - it was dead, even when connected to a power source.
The curious thing was that the last time it backed up was 2nd Sept. This was the day I got back from holiday, and I know I downloaded 10.8.1 that day. So it seemed to be ok backing up with 10.8 (although it was crashing all the time) - but it had never backed up with 10.8.1
I checked for a Firmware update, and sure enough on 4th Sept there was a Firmware update for the My Book - except I didn't know that and hadn't downloaded it.
I just took the My Book back to the Apple Store and exchanged it for another one, and this one came with the Firmware update from a week ago already on it. It seems to be working fine and is backing up now.
So I think ML basically killed my My Book... And perhaps something hadn't been working right there and had caused freezing etc.
So: All you guys with external HDs who are experiencing this problem, check for firmware updates. If that doesn't work, try stopping Time Machine backups and see if the freezing stops.