I have a similar situation, where I have to run the first GPU on my first 5970 at a lower aggression.  Otherwise the other 5 GPUs suffer.  It's not anything to do with the CPU, as the CPU has 2-3% utilization.  Maybe it's the drivers or SDK. 

Does WORKSIZE=128 increase your hash rate at all?  Also, try the first GPU aggression at 9 or 10.  One of my rigs likes the first GPU at 9, while the other one likes it at 10.