Incase anyone else reads this and wants to know why...
There can be a few reasons, ranging from heat output degrading performance (electrical components dont work as well at higher temps and there is a point at which they just about die off at a certain temp, for all components). Also, there could be the possibility that the core just hit its OCing capability which happens to be lower than the speed you have it at. What you can do is just lower the clock until you start to see a rise in performance again. Or clock up the speeds slowly from stock and find the best speed for your system.