Jeeeeesus... Just got the R9 390 installed. Huge problems. Works fine as long as no driver is installed, but as soon as the driver installation finishes, the screen starts going black whenever opening a window or doing anything that creates any sort of light or moderate load. Opening any 3D application results in instant black screen and loss of signal until the application is closed (Alt+F4 FTW

).
Just managed to get it to stop the
constant black screening by completely turning off PCI-E power savings in the active power profile. Mildly optimistic, I then tried running Furmark. Got a quick black screen flash within 1 minute.
So, folks, I guess it's time for my fourth AMD GPU. To be honest, I just feel like setting fire to the POS and move on with my life.
EDIT: Been googling around and apparently it's a pretty common problem going all the way back to the introduction of the 290/290X. Just like with the vertical lines issue on the 7970 and derivatives, people have come up with many attemps to "solve" the issue: downclocking, overvolting, creating custom power profiles, etc. Sometimes it works, sometimes it doesn't. However, again, it all seems to come down to either defective hardware or hardware that's simply not up to spec.
I'll see if I can verify the problem in another PC, then it's going back to the store.
EDIT2: I may have found a work-around... The issue only happens on the rightmost of the three DisplayPort ports (haven't tried HDMI or DVI though). Both of the others work perfectly. Additionally, even the right most DP port seems to start working if you unplug and then plug the DP connector back in while Windows is running. Looks like some strange timing issue. If the card works as expected except for this, I'll probably keep it.
EDIT3: I'm sorry for blaming you, AMD... Believe it or not, it was actually the
cable! This cable has worked perfectly until now, but the DP port in question appears to be running closer to the "edge" than the other two (and the DP ports on my previous cards). Pretty strange failure mode (being dependent on load), but replacing the cable with one provided with my HP Z24i monitor fixed the black screen issue completely. Time to game!