I'm having the same problems now. Sager 9860, Geforce Go 6800 with 256MB.
I removed my video card to find the thermal paste splattered and dried around the GPU, which I don't think is normal. In fact, I've never seen thermal paste in that state before.
Upon taking apart the metal case surrounding the graphics card, I'd discovered that one of the thermal tapes attaching the memory modules to the case was missing... so I suspect that particular module is the one that'd gone bad.
Whoever opened this thing before I did may have forgotten to replace the thermal tape there. I sure as heck didn't lose it because I'd never seen the inside of the metal case before. The heat sink had come off before, but never the metal case. The serial-numbered tape that I assume voids the warranty if broken was already cut before I opened it.
I have a warranty with Philips-Magnavox (which Power Notebooks recently did away with) but I refuse to send it back to them. They deal with MicroMedics, who f****d my laptop up the last time. I'd rather front the cost for a new graphics card than let them touch my computer ever again.