Definitely sounds like some sort of heat-related issue. A picture of what happens would be good.!@#$ it! okay, the D630 installed ubuntu 10.04.3 just fine. Connected to the internet. Ran 1 core at 800 MHz just fine. Left it sitting on the ubuntu desktop for about 30 minutes and it completely crashed the video. Ill try it again and get a pic this time.
From the feel of it, the GPU is probably hitting north of 100*. Might have to go in and manually edit the vBIOS to get it permanently down to 250 or so to cool off.