Need a little more info is it overclocked and has anything in the case changed did you put on take off the side of the case during testing. And not passing a prime test is not always a bad sign as the proseccor will not run flat out like that in any normal usage. What are the temps while surfing the net and say listening to itunes or something. Then what are they while gaming..? If these are fine and you rPC does not crash then dont worry to much aout it. Having a good case with good airflow will help in all cases.
I know you have solved your issue, good going, but this sounds to me like either the heatsink was not mounted correctly, or you have too much thermal compound. Are you certain you had the locking lever completely latched down into place? Sometimes they can be very stubborn about locking down into place completely.
I have always found AMD heatsinks to be quite good unless you doing anything more than mild overclocks.
A thick coat of thermal compound will transfer heat for a while until it starts to dry out, and as it drys out it becomes a better insulator than conductor.