Had this same issue on my old 5 card rig, it turned out to be a bad flash due to and in combination with a bad PCIE slot on the motherboard. Flashed it on another computer and re-inserted it into the PCIE slot via riser and it worked but due to the bad PCIE slot the system would randomly reboot. This is why I switched to all 4 card rigs.
it can be temperature or pcie problem, because both cards died when plugged in x16 port