Installing a second GPU kills the hash rate on the first GPU

[uranian]

I finally managed to get a 7970 up to speed today, so I then added a second card, a 5770 as it's low wattage and my PSU will handle it. As soon as I installed the second card, the hash rate on the first dropped by about 50%.  I've removed the second card, that made no difference, tried different slots with the 7970, even removed and reinstalled the SDK and Catalyst control centre, and I'm getting nowhere. I'm hoping at this stage to avoid re-installing Windows, anyone got any suggestions for what to try?