Mojave black screen on reboot after successful install
I have a (Mid 2012) Mac Pro with a replacement ATI Radeon RX 560 video card from MSI. This card supports Metal and is listed various places, including Apple support articles, as being compatible with Mojave. It has worked fine under High Sierra (latest 10.13 version).
I've installed Mojave (10.14.4), and the system boots and works perfectly... until I shut down or restart for any reason. Then I just get a black screen.
Safe Mode (holding down Shift while booting) DOES NOT WORK. There's no difference in behavior. Recovery Mode DOES work, and allows me to reinstall the operating system. When I do so, it installs fine and the system runs fine UNTIL I shut down or restart, at which point it won't boot again.
However, even Recovery Mode displays no video until I unplug the monitor and plug it back in. I know when to do this because the LED light on the monitor changes from yellow (no signal) to blue (signal present but screen remains black). This doesn't happen when I don't boot into Recovery Mode, and unplugging and replugging the monitor then does nothing.
With this third party video card, I always have a black screen (both on 10.13 and on 10.14 when it works) until the startup process is almost finished (bar reaches the end), so I get no feedback as to what's happening. However, I also have a USB mouse with colored LEDs. Normally, these LEDs come on when I turn on the Mac and stay on. However, after I've installed Mojave and the system isn't booting, these LEDs come on, stay on for a few seconds, and then turn off, which appears to indicate that the USB device has lost power. This leads me to believe that the boot process is failing almost immediately; it isn't just an issue of the system booting without working video.
I've tried booting with command-option-P-R; doesn't help.
I can't run Disk First Aid on my system drive from Recovery Mode. That's never worked on my computer, on any Mac OS version, because it says it can't unmount the hard drive. Running Disk First Aid from a working Mojave install reveals no problems.
I've installed Mojave three times now; exact same symptoms each time. I can reliably get it to work by booting to Recovery Mode and reinstalling the operating system but I can't get it to boot to desktop in any other way.
I could use a Time Machine backup to go back to 10.13, but I'd rather not, because my system was having severe problems with Spotlight that nothing I tried could fix, plus graphics issues with several games. Updating to Mojave immediately fixed those problems, except of course that my system only works until I have to restart.
This is very frustrating because it's clear that Mojave CAN run properly on my system. Any suggestions?
Mac Pro, macOS 10.14