Curious about your power setup:

1. Do you have the two PSU's on different breakers/fuse circuits? Same phase? They can't be on the same 15 or 20amp circuit can they?

2. You said you have three GPUs per PSU but risers on only one?  Shouldn't you be matching the 3 GPUs with same 3 connected risers to each PSU? Ideally the risers and boards should each be on the same rail of each PSU or it could cause some issues for any board that would end up being connected to two different rails or worse two separate PSUs. Assuming you don't have it setup this way, are your pigtails on the risers getting warm?