Not sure if this is a Mac Pro or Mojave issue or a combination but let me try to explain. After updating my cMP from Sierra to Mojave, my third monitor (which happens to be a Samsung TV) that is fed from the HDMI output from a Radeon RX 560 card no longer works properly. The display is turning on fine, but after a second it goes black, then back on. Then stays on for a few seconds, goes black again etc. Just to be sure it isn't a connection issue, I booted into my old Sierra partition, and sure enough, the display works fine. Any clues? This is really annoying and if this can't be fixed I will be forced to remain in Sierra, but I really would love to go Mojave because it has some Audio Units 3 features that I look forward to.