I would swap out your PSU with a more powerful one. I saw similar problems, and moved to a PSU with 50% more wattage (i.e. more stable voltage on those PSI-E rails), and these problems disappeared for me.
These setups are incredibly sensitive to high-quality power delivery.
It's on a $200 Rosewill 1000W 80 Plus Platinum single rail PSU riding on top of a $200 pure-sine UPS. I am getting a steady 12.01V at the power connections on the M-board. I would have gone with another Seasonic. But, this guy actually had better reviews for class I was looking at. PSU efficiency maxes out around 50% load.