Yep its definitely not locking up, it first I thought it was my power tripping, that’s how it goes off, I was leaning toward the motherboard especially with the pcie dropping from 16x to 8 and then to 4x. I know whichever one I pick to replace it will be the other.Lockups, freezing and BSODs are usually caused by software, CPU, PCIe devices, memory and motherboard.
But the issue you are facing is complete shutdown which will only be PSU, PSU Cables or Mobo.