I think I've narrowed down the issue to the motherboard. I am posting to see if others have similar issues before I attempt an RMA.

Per my system specs, I have 3-way GTX 670s in SLI for surround on an ASRock X79 Professional w/3930k.

This exact system works fine with NVIDIA's PCI-E 3.0 tool (force-enable-gen3.exe) with 1 or 2 GPUs in the system and the BIOS set to GEN3. For months really. But when I add the 3rd GPU, that's when I have serious problems.

The single consistent problem I can report:
  • The 3rd GPU is constantly not seen by Windows' device manager - as if it is not even plugged into the system.
  • Additionally, I get frequent failed boots with code "94" which is in the manual as, "PCI Bus Enumeration". THis only happens when there are 3 GPUs in the system.

Latest NVIDIA drivers, tried the latest beta, went back several versions. Re-installed Windows 2 times, went back to Windows 7 (where I had no issues with PCI-E 3.0 with 2 GPUs), etc. It all works fine with only 2 GPUs, but that 3rd GPU has a problem in this motherboard.

It is not the 3rd GPU
Before saying it is a bad GPU, take note that I have swapped out all of the GPUs in multiple slots and configurations. It does not matter: whatever GPU is in slot PCIE4 gets ignored by Windows when PCI-E 3.0 is enabled.

Since it is not the motherboard, not the PSU (I previously ran 4-way GTX 580s with this same PSU), each card works fine in 2-way in any slot with PCI-E 2.0, and whatever GPU I insert into PCIE4 fails with PCI-E 3.0 enabled, pretty tells me this is a motherboard problem.

Let me know what you guys (and gals?) think.