Hi,

This might sound pretty simple, but I usually see this get a lot of people when I am brought computers where the screen randomly turns off. When you inserted your GPU, verify that the card was fully clicked in. Like I said, I know it sounds basic but sometimes these cards can look like they are plugged in. Also, when you bolt it down, elevate it a bit when your tightening the screws. Sagging GPUs can sometimes cause the pins to not be seated very well.

Other things you can try and that have probably been mentioned before in this thread...
-Check your temps.
-Make sure you didn't accidentally plug the CPU pin into the GPU and the GPU pin in the CPU.
-Try another GPU
-I also seen you mention you got a new PSU. Make sure it is inserted in the correct orientation. The PSU can generate a lot of heat and you want to make sure that does not get dumped back into the system. Though, if the case doesn't allow for that orientation, having positive pressure (slightly, not overkill. EX 3 fans in, 2 out) in the case would probably help. Out fans being on top and rear of the case.
-And finally, delete the old driver from any old hardware.

Good luck!