Ok, I found out that only one power led of the two 6pin connectors from my GPU is lighting green, the other one is off. Both 6pin connector led's are supposed to be green when powered on. Disconnecting both 6pin cables turns both connector led's red.
So either the one green led is broken, or the graphicscard has actually a power issue, which would make sense, considering the blackscreens and freezes. Question is, why is the card able to boot into windows and "only" causing random blackscreens/crashes, when it shouldn't even boot with only one working 6pin power cable? Because when disconnecting the 6pin (the one that doesnt turn green), the PC wont even start.
Unfortunately I can't get my hands on a new gpu until early next week, so I'm stuck with theorizing.