Hi, I just got the i5-3570k, and while I know it's not amazing for overclocking, I figured I would try it anyway. Currently CPUID shows it at 4000 MHz. Same with ASRock extreme tuner and my bios. (Using ASRock z77 extreme4.) But when I run FurMark to test my GPU, it shows it back at 3400 MHz. I am curious, why is that?
You may have tuned on the Intel Turbo Boost Technology, which increases/decreases the processor frequency when needed. Also the Intel SpeedStep Technology may cause the processor frequency variation you are experimenting.
Usually when overclock people disable these techs.
Go into the BIOS, then disable these techs and see if there is still variation on CPU frequency.