I had the same issue, particularly after the last "HDMI Audio Fix Update" which was supposed to fix... HDMI audio issues. Well, I have a Xenon motherboard manufactured in July 06 which clearly does not have an HDMI port. I have never had issues with it until this time and I started getting heavy lockups.
It was a periodic lockup too, which was odd... I could go for hours playing games and sometimes within minutes it would hard lock.
So, I did the X-clamp mod to it along with purchasing an enhanced GPU heatsink (with Arctic Silver 5) with the heat pipe off eBay for $15.
Now it runs fine, even upgraded the hard drive as well so my speculation is that the "HDMI Audio Fix" was partly to blame for it... potentially putting more stress somehow on the system even without HDMI causing excess heat. Again pure speculation, but I don't think I might be that far off considering that there were a rash of broken systems after that last update. Maybe just sheer coincidence.
-X