I bought a Sapphire RX 570 4GB the other day and installed it into my PC. It replaced my old GTX 650. However, whenever I tried running Fortnite or Rocket League, the monitor would turn off saying there isn't video input and that it's entering sleep mode. It seems like everything else is working on the PC, just that the monitor enters sleep mode? Anyway, when turning the monitor off and on again, the whole PC reboots. This happens only when playing games, like in Fornite when I'd die and click on the Return to Lobby, at that specific moment it would turn black. No issues whatsoever when listening to music, playing movies or just browsing the internet. Temperatures of the CPU, GPU are 60-70 degrees Celsius.
However, since my monitor only has VGA, I had to buy an adapter from HDMI to VGA. I don't know if the adapter can be the cause, but if it is, surely it would crash the monitor other times, not just when playing games.
I did a clean install of Windows 10, updated all the drivers, I even tried the CD with the drivers that came with the GPU but same thing. The PSU is Chieftec 500W, but it's 4-5 years old and as far as I'm aware it said "80 plus at 225V".