Disable GPU throttling?

I have 3 x 6970 cards and 2 computers. One with Windows 7 and one with Windows 8.

Windows 7 has drivers 13.9 and Windows 8 has 13.12

If I run 1 card, or 2 or 3 on the Win 7 machine all 3 run at about 480 KH.

If I run 1 card on the Win 8 machine, it throttles, the fan spools up and then slows down, the activity drops to between 50 and 80%. The has rate drops also to about 300 KH.

If I add a second card to the Win 8 machine then that card runs fine at 470-480 KH but GPU 0 still runs slow with low activity.

If I swap the cards over, GPU 0 always runs slow.

So could it be a Win 8 issue?

13.12 issue?

Though I am running another rig, same motherboard as the other 2 also with Windows 8 and 13.12 drivers.

This rig has 5 x 6950 installed and runs perfectly well. All 5 cards at max usage / activity and all mining about 430 KH.

My CGMiner settings are pretty simple -I 18 -g 2 -w 256 -thread-concurrency 8192

I also have setx GPU 100.....etc in my batch file.

What am I missing? Is there a way to disable this throttling?
Modify message