I have 2010 Mac Pro. Just added a RX580 8gb Sapphire Pulse. Have three monitors that worked fine previously on Radeon 7950. 2 are 32" LG 32GK650F and one is a HP 24". Two monitors are hooked up with HDMI cables and one with Display Port. Sometimes 2 monitors will start - random which ones start. If the one that does not start is unplugged, and then plugged in a few minutes later all three may then come on. If all three are on and the unit goes to sleep usually ony two monitors will awake. I installed the latest Mojave system this afternoon and it does not seem to improve anything. I have used different cables and this does not seem to help. If I leave one monitor unconnected the two that are connected up always start. Can you make some suggestions?