the heatsink might not be properly placed, just a intuition, i would think it might not work if u had it wrong for a few mins at most.
also make sure the airflow is getting througho ur case in a correct flow
your lucky your mobo isnt equiped w/ c.o.p. b/c the threshhold is usually 70 C. I agree use arctic silver thermal compound....carefully remove all of the old pad or compound w/ rubbing alcohol and a pair of eyeglasses polishing cloth. Make sure you dont over apply the compound....just a thin layer in the center of the processor....if that doesnt work i would look at replacing the fan.
make sure there is enough compound on there too, have read about new people putting it on themselves, they put literally a paper thing layer and it just spreads out and doesn't give enough contact...