[se5a]

heat sinks are not that well attached right out of the factory, Overclockers prety much rip them straight off, lap them, then reapply the thermal compound and re-attach them.
out of the factorys they have a sticky foil pad which goes between the heatsink and the chip, this is dead easy and fast to apply, but is not as efficent as using thermal compound if applied correctly. (CPUs anyway, I presume it would be a simular story with GPUs)

you could possibly have a warped heatsink maybe.