Question Two different graphics cards failed in a row.

Apr 6, 2020
My PC ran for many years now without a single problem. But a week ago my display went black with the message that the Nvidia driver did not react and therefore I had to restart. This problem occurred every minute from that moment on and when booting even the boot menu, bios etc. were in wrong resolution and white lines / artifacts showed up on the display.

So I borrowed a GTX 970 from an old PC and everything was fine until Friday. In Rainbow Six Siege and Age of Empires 2 Definitive Edition the display went black again after about 30 minutes of playing time, this time I even had to restart the PC. Yesterday the screen stayed black even after a restart.
So I removed the graphics card again and replaced it with the original one. The PC started up but again with artifacts and constant restarting of the driver.

I switched the PCIe slots and bought a new PSU. But nothing works.
Since I tried 2 graphics cards and everything else is working I can't figure out which component is damaged and if both cards were harmed.