Ok I just let FurMark run to check the Temps again. 98% GPU Load, 22% RAM Load and the Temp tops of @69°C after ~5min. So I guess its not temp related.
Edit: Just ran Prime95 until the Temps were stable. CPU tops of @49/51/52/53°C. And thats with the CPU Fan @60%. I think we can rule Temperatur Problems out as a cause of my Problems.
Edit2: Also the Driver
Verifier is still enabled (2h24min) without causing a new BSOD until now.