You have 3 options really:

a) Forget about overclocking and be happy that it runs the way it does, which is outstaning BTW. Never would've thought this would be possible at all.
b) Get a new motherboard to get real Yorkfield overclocking.
c) Get to know how to mod GTLREF voltage on your board and probably some other voltages, like VCCPLL and so on. Mod your board and then try overclocking the yorkfield again.

For simplicity's sake, I'd suggest option b).