Check if you have the same symptoms and workarounds as I do: Could you offer a bios fix to a black screening RX480 8Gb (avoid RMA)?
You can also try ClockBlocker program (From Guru3D.com) to force max clocks and possibly prevent the issue? I'm still asking around if somebody could make a quick bios fix... There are already signed "mining bios" and "unlocked voltage bios" so maybe somebody could make it. Bios signature protection sucks to be honest.
I'm having the same issue with a new RX470 I just bought.
3D applications have no issues, but when idle/desktop browsing the card will black/purple screen and the system still runs. I'm pretty sure after some trouble shooting/testing that it is because the card is not stable at 2D clocks (or their lower pstates). Simple way to test is to run the gpuz render test while browsing the web or sitting idle at the desktop to force 3D clocks, if it doesn't crash there is a good case that is the issue.
I guess it could also be some driver issue, but at this point I'm leaning heavily on the cards being unstable.
The voltage theory definitely seems like a possibility - Haven't thought to keep a logging tool running while I'm using the computer, since it only happens about 2-3 times on an 16 hour day.
And there's one tiny issue with the voltage theory - It happened once while playing Cities: Skylines yesterday, which should definitely push it into a higher state at all times. :/
I've tested and confirmed it happens on both my 3570K setup, and our 4770K testbench setup, so it's unlikely to be a PSU power stability issue.
Some AMD response would be great at this point, since I'm kind of left with no options to RMA here.