Update
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Seems it's not my power supply causing this. I bought a new one, 1000Watt and it still has the same issue.

Whenever I try to run games or more demanding things, it will crash eventually. I can surf the web for hours without ever crashing once though.

So what's next on the list? Bad RAM memory, 3d card or bad motherboard??? Bad motherboard seems very unlikely don't you think?

Temperatures are all normal, nothing is overclocked, voltages are fine and all ram sticks seems to be recognized just fine.

There are currently no graphical errors running Windows normally, although a few days ago it did display weird graphical errors in Photoshop. That seems to be gone now, perhaps because of a recent driver update.

Whenever the PC reaches a certain load, it will make the screen go black and eventually it will crash.

What the heck could be causing this?

edit: Ahh, sometimes when rebooting, it does make beeping sounds a bit longer than normal. Seems to be the usual graphics card beep (1 long, 3 short, 2 short). But... running 3d programs or games isn't an issue as long as I do not go full screen (that will cause my video card's fan to blow at full speed, causing a crash so it seems).