First thing to do is run the computer with the case sides off. If the CPU temp drops a lot, you have to improve your case cooling...install some case fans and clean up the wiring.
If the CPU temp doesn't drop, the heatsink is probably installed wrong. It should be sitting nice and level on the CPU die. Recessed part of the heatsink should be over the raised part of the socket. Metal clip should make contact with the heatsink directly over the CPU die.
If the heatsink is installed properly and your case cooling is set up right, they take a look at heatsinks the come with a 80mm fan on them. The SpeezeFalconRock is a good heatsink.

Cricket