AM4 PCIE instability with 3090FE and riser cable


Jun 6, 2003
Upgraded my daughter's computer with leftovers from my recent upgrades - went from a very small (sub 5L) sff with biostar x370gtn, 3700X, corsair lpx 3000 cl15 ram, and gtx1650 with a short pcie riser cable, no issues with the system, running reliably for almost 3 years. I put the board and storage (pcie 3.0 nvme boot drive and sata solid state storage drive) in the new phanteks evolve shift XT but swapped the 1650 for a 3090FE and the 3700X for a 5950X. I also moved from the 220W psu in the old unit to a phanteks 750W unit. I updated the bios on the old system and made sure it all worked, then built the new system and configured the system via bios (basically loaded the docp ram profile).

It loaded into windows fine and I loaded the latest chipset drivers and nvidia drivers and rebooted the system. It immediately started crashing at various stages, sometimes letting me get into a game (used minecraft rtx to test), sometimes barely making it to the desktop. No blue screen, just a black screen or lockup with no mouse movement. I did tons of troubleshooting, including swapping the old 3700X back in with no improvement, and finally landed on the pcie bus speed. Downgrading from pcie 3.0 to 2.0 fixed all stability issues and it's running fine. Now the Shift XT does have a riser cable, there's no way to run a video card in it without it, but it's a pcie 4.0 cable and the board is only operating at 3.0 - plus it was running fine with a (albeit shorter, but only pcie 3.0) riser cable in the old system with the 1650. I haven't swapped the 5950X back in yet (it's a bit of a pita in this system as the board has to come out) but plan to if I can figure out the pcie issue. My initial thought is it may be the riser cable or the board can't handle the new setup. I have another riser cable that will reach I could try, but I have no idea if that one is any good, either. Is there anything else anyone can think of?
Risers are incredibly flaky in general. I had bought a bunch of the PCIe x16 shielded ribbon risers and x8 shielded ribbon risers on Amazon (highly rated, not inexpensive) for a build with a bunch of RTX 3070's. Turned out about half of them would only function at PCIe 2.0. The machine would boot at PCIe 3.0 x16 but would hard lock or reboot frequently. I bought a bunch more and mix and matched until I was able to get all 6 cards running at the correct PCIe generation and link speed. I would for sure start with another riser and see what happens.