I have two XFX R9 295x2 GPUs plugged into the 16x slots on an MSI 970A-G46 motherboard.

The system will boot up and operate fine. Mining ethereum - I can load the DAG and the second that it starts mining with the stock clocks on the GPUs, One of the cards will litterally power down ( lights out)  while the other crashes and gives me a black screen. Upon further testing, I can downclock and all 4 GPU cores will run, BUT, I can only run the GPU's at around 650Mhz GPU clock and 650Mhz Mem clock for all 4 cores. If I try to increase any further, once again, I'll crash.

If I just use one R9 295x2 card, I can go stock clocks, plus over clocking.

The motherboard is on its own 650W power supply while the GPUs have their own 2980W power supply, so the GPUs have plenty of 12V, I'm just wondering if the PCI-e interface is the limiting factor that is causing the cards to crash... IE: not enough current on the buss

This motherboard is a gaming motherboard, however I have never been much of an overclocker since I really don't play games, so going into the BIOS to tweak settings is abit new. Can I change anything in the BIOS to make this run the two R9 295x2 GPUs???