did you try to install back your PSU? I am suspecting a PSU or a riser issues. so first in, try to install the faulty GPU alone using the old PSU in the PCIex16 slot.
It happen when I was if my best PSU and was using direct in the second PCIe slot, when I try to use the faulty GPU alone I dont get video in neither my PCs.
Your card cant run with power but run. One gpu need service reflow or reball gba.
You mean the GPU is "ok" but have a problem that can be fixed with reflow or reball gba?