Far out mate. A little punctuation would go down well :).
To the OP. First off, no need to be taking off the heat sink just yet.
Take the side cover off the case and have a look at the fins on the heat sink. If it is full of dust, clean it out completely using a small artist's brush, or some compressed air.
If it is not full of dust, check that the fan is turning when you start the pc up.
If you still have problems, post back.
crunchie
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There is absolutely NO need to take sandpaper to the heatsink, unless it is not flat. ie; convex or concave.
If one ever needs to, then clean the surface with Isopropyl Alcohol.
That will remove all the original TIM (thermal interface material), so that one can apply it new.
crunchie
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