Could be a number of things:
- Bad case airflow
- Bad contact between CPU and HSF
- Too much or too little thermal grease
- Bad temperature diode in the CPU
- Motherboard incorrectly reporting temperatures
- CPU overvolted (this might be because of a bad motherboard PWM)
Can't think of any more for the moment.
for case airflow, how much are temperatures affected by having case open, and case closed?
Re-apply thermal paste (try some better thermal paste too, if you have any), and then make sure the heatsink is firmly in place.
Lower voltages slightly in the BIOs, see how temperature affects it. Also, get some software to measure voltages. Might not be very accurate, but worth a try to see how much voltage fluctuates by.
Try a BIOS flash if there's a newer BIOS, that may help if the motherboard is reporting tempertures incorrectly.