Bit of a mystery: I purchased the Ableconn PEXM2-130 & 2 Samsung 970 EVO Plus 2TB blades + heat sinks. I always test everything first on my back Mac Pro 4,1>5,1 currently running macOs 10.14.6 & OpenCore. Eveything worked, so I installed the card on my main Mac Pro 5,1 running macOs 10.13.6 (Boot ROM 144.0.0.0.0). No problems, so I copied my FCP X caches and libraries from my scratch SSD to one of the EVO's. Turned the computer off to remove the scratch SSD and the computer wouldn't start up--I would get the boot chime and after about a minute or so, the power would shut off. Transferred the Ableconn back to the Mac Pro 4,1>5,1 and now it did the exact same thing.
FWIW: It was VERY hot in Los Angeles yesterday and I have no air conditioning. I keep an extra fan on the floor of my apartment office to circulate air around the computers. I let the card and MP4,1>5,1 sit until 10pm, and tried again and it booted. I left the computer on overnight and no problems, it's running now as I write this. I installed iStat Menus & Macs Fan Control to monitor temps, and sitting idle the NVMe's are 47C & 48C. Room temp is approx. 30C/86F.
I'll probably try moving the Ableconn back to the MP5,1 tomorrow but do you think this could have been an overheating issue? Is it something I might run into frequently?
FYI: The Ableconn sits right above my RX580 GPU in the Mac Pro 5,1 and above an RX560 GPU in the Mac Pro 4,1>5,1. The OS of each computer is on a Samsung 860 EVO SSD in PCIe slot #4.