After tinkering around with my Graphic card in other PCs without being able to pin-point any errors on it, i finally threw in the towel and ordered a new 750 W PSU, and i'm happy to say it fixed everything.
I have been playing under full load for +8 hours at a time, temperatures not exceeding 55 degrees C for a couple of days now, so i guess that was my problem.
So if anyone else is experiencing similar problems as i had, it might be your PSU
Now.. how do i close this thread and select best answer? I don't see any obvious buttons to click