Random short black screens

I very frequently get very short black screens that just come from nowhere and disappear in about 1-2 seconds. Usually followed by several more. Then stops for som time. And then at a random time comes back again.

I honestly got no clue to why this is happening. I've noticed it happens if hit my pc so it shakes a bit. But the black screens occur even without me doing that.

Tried switching my hdmi cable 3 times... Nothing changes. I am running a dual monitor setup which might have something to do with it, as it doesn't happen when I only have the blackscreening monitor plugged in.

For anyone interested, my monitors are: (Main monitor: Acer G246) (Secondary monitor: Acer G245) They're basicly different gens.

I have tried new/old drivers for my graphics card but nothing helped there either.

Thank you for any help on forehand!

Assuming you did the basics (updated drivers and OS, Malware scan, device manager says all is well in the world etc)

This sounds like a Graphics Card issue and the Black Flashes lasting 1-2 seconds could be the drivers recovering the card from a fault, or the drivers itself.

Only real way to test this is to use a different card for a while and see if that solves the issue. 

Unfortunately I don't have the money for a different card atm. Atleast not one that is worth buying.
What I feel might be a source is the dual monitor maybe? As I stated, it doesn't happen when I got a single monitor plugged in. Only when dual. 

You haven't stated your GPU anywhere, if it is a lower spec one you could be overloading the card and causing it to crash the drivers.

I'd purge your drivers and re-install them and see if the fault continues.

I've got a Nvidia GeForce GT640