My screen goes blank / black all the time, even more so during games?

Hey guys, I recently purchased a newly custom built PC on ebay and a AOC 2236 monitor seperately. The problem is that my screen goes blank / black all the time, in games it's much worse, the screen goes blank every other second.

I tested the monitor on my old PC and it works fine, even in games.
I tested the new pc with my old monitor and it works fine.

so...I'm really confused.

I've tried:

- updating graphics and monitor drivers
- upgrading from xp to vista
- all different types of resolutions
- all refresh rates

Any help would be greatly appreciated. Thanks