I can't even get the bios to post. I swapped all the parts minus the mobo from bad rig to good rig, and they worked fine. so i'm guessing I have my answer
This may have been suggested before, but I'm lazy and don't like to read so I'll just toss this out there and take the heat later.
Have you tried adding power to an ATX4P connection on the motherboard (or molex 4 pin connector if it has one) to supply additional power to the PCIe connections? Even if the risers are separately powered, sometimes the board can't produce enough juice to the PCIe slots to support GPUs in all 6 positions.
I have a Gigabyte Z170x-UD5 TH mobo that had that bootloop issue that drove me nuts for a few days straight. It wouldn't run more than 5 cards when there are 6 slots (that was with all gtx 1070s) so I finally discovered the purpose of the ATX4P connector and added a SATA power cable to the board and now it boots fine with 5 1070s on risers and a 1080ti in the last onboard slot.
Sorry if this is redundant info.
I'm only running 3 cards so idk if that is super applicable but if nothing else works I'll see if that helps. Thanks for the suggestion