There is no need for custom Bios , P1.20 is fine.
If you use powered risers do not use 4 pin Molex onboard.
You can use the iGPU or not up to you, i'm running 5 and 6 cards + igpu with this board.
Disable Render stand by in mainboard bios.
Disable sound in mainboard bios.
Is your Chipset driver installed?
Remove GPU drivers in safe mode with DDU.
All cards are connected through risers? Or do you plug in one card in PCIe 16x?
Thanks for the swift reply
Neither BIOS (1.20 or 1.30) seem to help
iGPU makes no difference
Render standby and sound are both disabled
Chipset driver up to date and installed
I've fully cleaned with DDU twice now
All three cards through risers on PCI 1,2 and 3, also tied connecting the GPU in PCI2 directly to the PCIe 16x on the board.