It could be bad seating. I have never seen a temperature jump like that with any cpu I had. Granted I haven't used the 4800+
Go here http://www.arcticsilver.com/ins_route_step2amdas5.html
and read how much AS5 you actually apply to the cpu. It is so little that a normal tube could probably do at least 100 cpu's. If you apply more the AS5 becomes an insulator. If you gooped it on and smeared it over the cpu that could explain the huge jumps in temperatures.