Overclocking stability with multiple cards and CGMiner

[phluxx]

I've noticed that in CGMiner my second card (GPU1), always seems to perform worse. 

On one rig I have a 6950 and 7870.  When the 6950 was alone, I was able to clock it at ~940Mhz core and it was rock solid stable.  Since I've paired it with the 7870, I'm unable to get it stable at the same voltage at anything higher than 880Mhz.

I now have a second rig with two identical 7870s in it that is having the same problem.  The second card, under GPU1, dies at overclocks that should be stable.  I realize there is variance from card to card at what is stable, but it shouldn't be a 40Mhz difference between identical cards?

Any advice?