Check all your power connections, it could just be a loose cable. Or a bad cable, you could try swapping cables around if they're modular.
Make sure you have all three mobo power inputs connected (24 pin EATX, 4 pin ATX12V, 8 pin ATX12V).
You have two fat (250W) GPUs, so you should also connect the optional 6 pin "black" EZ Plug (extra power for mobo PCIe slots). It can't hurt to also connect the 4 pin "white" EZ Plug (extra power for mobo DIMMs) if power delivery seems iffy.
A good excuse to ensure you've updated to the latest firmware and software for your board.
You should probably confirm your AC power source is clean (using a
receptacle tester,
Kill A Watt meter, DMM, or similar device). You could have "dirty" power mains, sporadic brownouts, excessive impedance/harmonics, etc. The only workarounds - aside from a complete electrical upgrade in your building - are to move the equipment to a different (clean) power circuit or to include some sort of uninterruptable/backup power supply, power conditioner, isolation transformer, or somesuch.
Your 1200W PSU should be adequate. Unless it's dying. Swap with a working PSU from another machine if you can, for long enough to determine the source of the problem. If the power source and PSU are perfect then your mobo might be crapping out.
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