I had a very similar issue and couldn't figure it out for the life of me for the longest time. I am running 4 cards also with 2 PS, but they are 290's and 7970's.

What fixed my issue was plugging the modular PCIe power connectors into different slots on the power supply. All the pci-e ports on my PS are powered from the same 12V rail so it really doesn't make any sense that would fix the issue, but it did.

Are you running 2 cards off each PS? Can you take pictures of your setup to verify everything is hooked up properly? Do you have a device to measure the power being pulled from the wall?