I have an MSI board and Cool 'n Quiet has never seemed to work.
I may be wrong, but I think there might be an issue with the nForce chipset and AMD Cool n Quiet... I have seen a couple of BIOS updates for my board that claim to sort out the problem with certain CPU's. Might be worth Googling it.
Also, have you checked your running processess, just in case you have something running that is eating in to your CPU time? It might be stopping it triggering the slow down.