I bought a dead xbox off of ebay for £10 the other day - listing specifically said something like "Been fixed 4 times - won't stay on for more than about 10 mins"
I actually bought it for the case, figuring I can mod THIS case, then transfer my xbox into it - so I don't have to be without the xbox while doing the modding - so as a donor of sorts.
I opened it up and stripped it down - there was a MASSIVE amount of TIM all over and around both the CPU and GPU and there are 4 chips around the GPU on the underside which I know need to be cooled which had no cooling on them at all.
I thought - I wonder.... maybe the 4 repairs it's already had were all done utterly incompetently....
So I went to it - dremmel attack on some old heatsinks I had lying around - MUCH more sensible application of TIM, and good clean out etc. Re-seated both heatsinks on the CPU and GPU using brand new x-clamp replacement kit (£2) - and it's been working 100% for about 2 1/2 hours now and the fan isn't changing pitch at all so I'm assuming it isn't getting hot. An hour of Dirt and 90mins of Gears of War 2 later, and it's still just fine n dandy
Got me an xbox for £12 and it works like a dream