EDIT: Skip to post #12
Three days later…it was not a problem with the expansion card or the 970 Pro.
TL;DR
My X570 Aorus Elite was not stable with an NVMe connected to the chipset while a powered USB hub was connected to the CPU lanes. Moving the powered USB hub to a port on the chipset resolved the problem.
Actual Symptom
I did a fair amount of CPU/Mem stability testing when OCing cheap Micron E die to 3666MHz CL16. At the time, there was one NVMe connected to the CPU lanes. For this issue, I loaded some dummy files onto the drives and tested with Aida64’s System Stability Test:

It was not immediately apparent that one NVMe on the chipset lanes was an issue, but Aida64 test proved otherwise. It never made it past the 5 min mark before the GPU shutdown.
It was very obvious that a second NVMe to the chipset was a problem. The mouse cursor would lag, and I couldn’t even load Aida64.
Things I Tried
- Stock settings by clearing the BIOS
- Forcing PCIe 3.0
- Three different PCIe 3.0 drives (970 Plus, 970 Pro, Sabrent Rocket)
- The board’s M.2 slot vs PCIe x4 expansion card
The Clue
After the GPU shut off, the mobo continued to run the fans and RGB. Pushing the power button turned off the fans, but the RGB only dimmed to a faint glow.
I thought it entered a weird power state that was affecting the tests, so I flipped the PSU switch. It continued to glow. I pulled the power cord. It continued to glow. I pulled the entire power strip. It died.
I have a number of USB devices with auxiliary power - BenQ monitor hub, Ohaus scale, multi-function printer, 3D scanner, but the RGB was somehow drawing power from a TP-Link 9-Port USB 3.0 Hub.
The Solution
The hub was connected to one of the four, blue USB ports that ran to the CPU. Moving it to a red USB port on the chipset solved the problem. I’m over an 1:30 with the USB hub and all three NVMe’s playing nice together.

CrystalDiskMark reports all the drives run at their spec. I was also curious as to the transfer speed between the drives This is what robocopy reported from loading the dummy files onto the drives for Aida64’s read tests:

PS
The powered USB hub solved another issue, namely the poor circuitry on my Blue Yeti.