After installing my new graphic card and testing it for a few minutes, the PC always rebooted.

First thing was to run some stress-tests to verify if it was the GPU. I used FurMark for this. It even includes a simple utility to see your GPU stats (like temperature).

And I could verify the issue. Always when reaching 69° GPU temperature, the PC rebooted. Since GPUs should work with temperatures above 80° or even 90°, this seemed to be a hardware issue. It seems like the sensor was broken and showed lower temperatures, so in reality the GPU temperature was much higher but since the sensor reported lower temperatures, the coolers didn’t run fast enough to cool the GPU down and after some point, the maximum GPU temperature was reached and it shut down.

After getting a replacement card, it finally worked without crashing and I’m now able to enjoy fancy new graphics.