I bit the bullet and got the I/O CREST SI-PEX40129 and dropped in a Samsung 970 Pro 1TB that I already had. I have the same symptoms that you do. This is a 2009 Mac Pro flashed to 5,1 and boot rom 141.0.0.0 with that card in slot 2, and an MVC flashed GTX Titan in slot 1. Copying a number of large files, it eventually hangs. Tested in both High Sierra and Mojave. After it freezes, a few seconds later it appears that a watchdog timer in the machine recognizes it has frozen and reboots the machine.
I could always make it hang by running Black Magic Speed test for maybe 5 minutes. When it was working, it was giving 2600+ MB/sec writes and 2900+ MB/sec reads.
I wasn't real happy with the build quality on the card. It uses thermal pads above the blades, but the heatsink is a little too close to the blade and was pushing down hard enough to bend it a bit.
That, and the included screws for the NVME standoff were the wrong size. Luckily I took a couple of screws out of an old Macbook, and they fit the standoff. They were also very sloppy in attaching the thermal pads to the heatsink.
As another person mentioned, fan noise was... meh. Nothing you would want to add into a sound studio though.
I am beginning to wonder if this card should be removed from the compatible list in the top post.