Ive been unsuccessful running more than 4 cards per rig.
My solution was to buy more motherboards and run 4 cards per rig. This has increased my cost, but there hasn't been anything yet i've tried that has worked.
I've had as many as 8 in a rig working fine, but HBCC enabled only on 4 of them, and i've tried this across multiple motherboards, different things.
I have one potential solution i will try when i get time to take a few rigs offline for a few hours. We'll see...
Use this:
https://bitcointalk.org/index.php?topic=2315166 or write it yourself