If you removed then replaced the heatsink you should have thoroughly cleaned both surfaces and then applied new, high quality thermal transfer paste or compound before refitting it.
You are obviously geting poor heat transfer, and with such a hot-running processor that could get to be a disastrous problem.
See here for some ideas and tiops:
http://www.arcticsilver.com/arctic_silver_instructions.htm
Artic Silver 5 is the best such paste by far, in my view.
Catweazle
Grandad
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