Have you tried to simply change the risers? One bad riser can make an entire system act funny.
I'm guessing you used DDU to remove and resinstall the driver? Patched it so it sees modded cards and all?
Have you tried changing RAM sticks?
I have tried to plug the graphic card directly into the motherboard. same result

UBUNTU 16.04 Server with AMDGPUPRO and stop suffering

tried linux, no money.
install amd driver 17.7.1 (no mixed drivers) then apply the pixelclockpatcher (renamed to only sign the driver), then reboot.
I doubt its a driver problem, system does not boot up, meaning I don't even get past any bios/uefi screen.
Sorry if this has been proposed before but a system that doesn't boot up usually comes down to:
-RAM
-Faulty PSU/power cable. I had some second hand pcie -> molex cables I bought off aliexpress that fried a riser. When I would start the rig up, I'd here a "click clack" in the PSU.
-Motherboard
Drivers are probably not the cause since you're not even getting to the windows loading screen.
Have you also tried to disable everything like Audio, Ethernet etc. in the bios?
I'm asking because one motherboard in particular was a giant pain in the ass, it was an asrock anniversary, one with 6 pcie slots on 1150 chipset. I got it working once with 6 GPUs, but had to disable everything in the BIOS (audio, ethernet...). Then decided to do some changes, can't remember what it was, I thing I did a DDU to update some drivers, and never again was I able to get 6 cards working on that thing.