Not sure this will be relevant but your system of 12 cards using one 850 psu for the risers is how I killed two psu. I only had 7 cards running and it was too much for the Sata channels to handle. One psu will be good for 6 sata devices so I think you are browning out the main psu. Once I dropped back on card count, no more problems. In the end, I gave up on the Server psu's and went with dual 1000w psu's.

thay