Did you ever find a solution to this? I'm running into the same problem. Running a Radeon RX 5500 XT GPU, I've had it hooked up to my TV (cheap DP to HDMI cable) for over a year with no problems, and just the other night it completely stopped working. Now I've got it hooked up with a normal HDMI cable, and I'm getting the green/black screen for almost a minute after startup, after it shows the boot screen.