1) Hardware is not overclocked
2) Ensure that the machine is adequately cooled:
(Temp 1 is CPU)
3) All drivers are up to date
4) BIOS is up to date
5) Windows is up to date
6) RAM & HDD: No errors occured
I found out, that the graphic card gets really loud when the computer restarts by itself. Therefore, I tried to move the graphic card back- and foward in order to check if there is any loose contact.
The computer restarted. I didn't get a BSOD - it just restarted and said there was a BSOD after logging in.
After that I disconnected the graphic card from the motherboard, removed the dust and connected it again. I'm not sure if it's the problem of the graphic card or the motherboard. But I have no BSOD so far.