2 1300W PSU (each)
I think OP was using 2 x 1300 Watt PSUs for each rig, in another word 2600 Watt each rig, that is right OP? If it right, a bit chance that power is the issue.
(They think you were using 1300 Watt for each rig)
OP
Have you tried plugging the cards one by one? And find out how much plugged cards that giving you untroubled rig. If possible use the same wiring scheme with the rig that was running smoothly (GPUs, Risers and other wiring schemes.)
In other thread, I found someone has troubled with the same mobo causing by un-mounted additional PCIe power (the two Molex connector) May you have to check it. (but I'm not sure about it) Since we are using a powered riser nowadays.
This is also good advice must you try:
DDU to remove it all and disconnect all GPU's. Install one at a time and reboot after drivers install for each GPU. Boot back up and verify GPU installed successfully. Then and only then shut down and install next GPU. Repeat cycle until all are installed.