I've been experiencing this on my 3 year old RX480 as well, searching around seems that many people experience this. Most likely a driver issue. I have a two monitor setup and sometimes either one of my screens would go black for about a second and go back on. There are no errors generated in windows event viewer and no driver crashes or disconnect sounds or anything.
Do note, I've started experiencing this this year and not beforehand. Cant quite point at a time however.