Couple of questions:
1) What PSU?
2) How many video cards? And models?
1) Rosewill BRONZE Series RBR1000-M 1000W
2) 3x PowerColor AX6950 1GB
Knew right from the title you had 69xx cards running..
Go to catalyst control center, write 'overdrive' in the search field and disable PowerTune by setting the slider to 20 on each GPU.
Otherwise the card will constantly throttle down and go back up in an attempt to 'save power' (a mechanism which might work for gaming but is useless for mining as you want constant peak performance)
Why does it always seem that the 69xx cards have weird issues! Underclocking in Linux, Opening GPU-Z while using the OpenCL miner hard locking windows, and now this! I'll give this a shot when I go on my lunch break. Thanks!