Sounds like the CPU is using something called Thermal Throttling - in effect, if the cpu gets too hot, the cpu will slow down to cool itself, to avoid damage. AMD cpus (well, XPs) dont have it, instead they just restart the pc to cool down.
So get a desk fan (one of those ones you have around the house for when its hot) take the case side off, and switch the fan on, blowing ONTO the heatsink. Try running 3d mark then, if its better, you know the cooling is the iissue, which it sounds like, as that is really quite hot imo.
Sounds like the cpu may have not got enough (or any) thermal compound on the coore, so it gets much hotter, as obviously the heatsink is on.
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My love’s subliminal