Alienware Area-51 M7700 Grey Screen?

I just bought a used Alienware Area-51 M7700 off of eBay for $200, the guy said that the GPU might be bad. I was just going to strip it for parts but the darn thing booted right up! It did go dark after about 15 minutes so I pulled the GPU and the port on the MOBO had some gunk build up so I cleand it, reinstalled the GPU along with the HSF & artic silver. The thing ran fine for over 5 hours; including a full 3dmark06 run and a service pack 3 install. Then it just went black; I pulled the GPU again and it was HOT, the thin layer of Artic Silver I used was not making contact with the Heat Sink so I put on a dab more, re-seated the GPU/HSF and it fired right up only to go to a grey screen shortly after. I went to bed, tried to start it this morning and Grey Screen again. I am going to pull the GPU again tonight clean everthing and re-apply new paste to see if I can get it to boot. The GPU is an ATI Radeon M X800; if anyone has any thoughts on this I would appreciate it.