Weird screen flickering when starting CS 1.6

[IroNNNNNNN]

Hi

Since a couple of days I have a strange issue. From the moment I start CS 1.6 I can see the start screen but then it starts flickering (black, no other colors).

I've looked around on the internet, saw several cases, but none really applied to me.

I've been testing and found out this:
- I suddenly can't run CS:GO anymore either, if I click 'PLAY' it seems to be starting for a few seconds but then it just stops loading anything.
- I can run any other steam game without problems.
- I can run any non-steam game without problems.
- I can view movies without problems.
- My ASUS AI Suite tells me there's no updates needed.
- Geforce experience tells me there's no updates.
- The flickering does NOT happen when I hook up another screen.
- I'm using a lenovo screen, but the site keeps giving me errors so I can't check for updates on drivers (even though I've never used any screen drivers).

So the fact that if I attach another screen could tell me it's a hardware issue, but then again running anything else but the CS-franchise works fine. So that kind of says it's not a hardware issue...

It seems pointless to me to add a screenshot, just push the on and off button of yours and you'll see what I see :P .
I have short flickers, but there are also long ones. It just goes all black (but my screen does not go into standby-mode).

Also giving my setup seems a bit useless since everything else is running smoothly... But if you need anything just ask. I'm hoping anyone is just able to help me out here.

GrTz