I currently use Catalyst Control Center to overclock
I found your problem.
It's generally better practice to overclock via the BIOS. I don't really have a good reason but that's just what I've heard and it makes sense for stability reasons I guess since it's overclocked on boot and not something that's controlled by a program. "Lower level" I guess would be a better way to put it. Also what kind of overclocks you can get vary from chip to chip depending on the cut of silicon the chip is made out of. I easily achieved 4.0GHz on my 955 while I know some people struggled with a 965 (955 = 3.2 stock and 965 is 3.4) to even break 3.8GHz. Just depends really.