Where do you get your core readings from? Also, make sure that your drivers are up to date. The other possibility (maybe a long shot) is that the card cannot go to full speed because your PSU is too weak. Complete system specs?
My core readings come from msi afterburner and gpuz, I have the on screen display in afterburner set to show clock speed in game. I have also tried it with furmark and I have done multiple runs of 3dmar at different clock speeds and they all come out exactly the same. I am on the 10.6 drivers and have tried 10.5. I doubt that it is my power supply as I was running two extremely overclocked 9600gt's in SLI before this. My system is a Core i3 530 @3.97, CM Hyper 212+ heatsink, 4GB Adata DDR3 1600 @ 1900, AsRock p55 extreme motherboard, 1 dvd burner, 1tb Samsung f3 HDD, 550w Antec True Power Trio, Sapphire Vapor-x 5750, 3 120mm case fans, 140mm fan, and a 80mm fan all in an Antec 300 case.