I just use the xclamp fix on all xboxes that come my way.. I run a small business on craigslist and charge 30 bucks a pop, make money so easy, there's tons of people who have the RROD.
In my experience, it seems to be some sort of a weak solder joint on the GPU chip. I just overheat the chip, turn it off, apply new thermal paste which is way better than the junk they put on there in the first place and then tighten down the gpu even tighter than before. I also think the xclamps don't provide an evenly space pressure and sometimes aren't on tight enough. Varies from box to box.