I too am having the same problem.
Tonight I was going to go over to my local Apple store, and I use the term 'local' loosely, but I am not prepared to have them send it away for three weeks, and then get it back with a brand new motherboard and logic board, but my hard drive gone and still having it shut down every 20 minutes.
I have the AppleCare thing, which cost me like $400. If I go over there, will they give me a whole new computer right there?
I have Boot Camp installed. Does that matter?
I've read that if you wrap a piece of electrical tape around the wire under the heatsink, that fixes the problem, but it will void the warranty.
I hear some people saying the problem is the logic board (or the motherboard), which I don't think it is. This is because, at least in my experience, no matter what problem you're having, if the Apple tech can't fix it over the phone, they always tell you it's the logic board. Am I right?
What should I do???