Clock speed moving about under load

[thecremeegg]

Trying to run the heaven benchmark and according to CPU-Z the core speed is at 1604MHz then it jumps to my overclock at 4GHz then back to 1604 again for a bit. What's causing this? Surely speedstep shouldn't be affecting it under load?

I have a 2500k

 

[setter]

Heaven doesnt work the cpu that much, as it's primarily gpu power thats utilised in it, therefore it will be speedstep doing its thing.