Lets dissect this so you understand better.
kHz :
So i was playing a competitive game on CSGO and suddenly my screen went black for few seconds and after that my FPS dropped to 80-100 (before that event i had 200 constant fps), after this happened my fps are blocked to 80-100 and they wont go up..
You got a new monitor that has more pixels (1920x1080p). You were playing CSGO and the computer screen went black. A few seconds later it came back on. What this tells us is your graphics card crashed due to instability. Now when this happens the GPU will 'reboot' itself to a lower clock. This is why you are going from 200fps to 80-100 after the black screen.
kHz :
That's kinda weird to me since i always had the same FPS and now it suddenly goes down to 80-100, and it's not going back to normality. Please help i already tried a lot of things but seems like i can't solve this. If it can help the only thing i changed was the monitor (benq xl2411z), but i played with it for few hours and it was all fine...now i just can't solve this.
Okay, now remember how I told you your old monitor had less pixels? Fewer the pixels, the less information the GPU has to process. With this new monitor your GPU is working harder because of the extra pixels. And that extra work is making it crash.
Now I know you said you lowered the graphics and are achieving the same fps rates. What you need to know is that, Some graphical options don't have the same impact on fps. Lets take textures for example. Lets say you were playing @ medium settings with your new monitor and wanted to increase your FPS so you dropped textures to low but you saw no increase in FPS. That indicates textures is not demanding enough to effect performance. So you need to keep reducing settings to find the ones that have a real impact on your FPS.
So what does have a big impact on FPS?
Anti- Aliasing. Reduce that to 0.
Turn shadows off.
Vsync: off
Hope this helps KHZ