I am wondering, if you ever tried to replace the power-cord "GPU to power-supply"? My defected cable was working for months, but I always got some flicker on my monitor. I thought my screen is just no synced. It helped to switch off switch on the screen. But apparently the cable was not able to...
I had the same issue: black screen randomly over months - especially while gaming. Yesterday I figured out, it was my power-cable from GPU to power-supply. I replaced the cable with the stock I got with GPU. Since then no problems anymore.