Keep in mind make sure you put just a small drop of the thermal compound the cpu. It's only a conductor of heat that transfers hear from the cpu to the heatsync. Again if you are sure the fan is clean and the Heatsync itself is clean and it's still running hot like that just from turning it on. You have a heatsync problem.
I am going to advise that if infact even if replacing the heatsync and you are still getting an overheat there may be a motherboard problem again I doubt it but I never over look anything.
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