You stated that the gpus are all on molex connectors, are you not using any of the PCI connectors from the power supplies? According to the PSU specs, you should have 4 x 6-Pin, 4 x 6+2-Pin per PSU.  That is enough to connect 4 gpu to the pci connectors and two to molex. Not sure if that would make a difference but it might be worth a try.