PC crashes/turns one monitor to one solid colour and then either it fixes itself and the running program may or may not crash or PC completely crashes

Hey, my PC sometimes crashes (now it happens way faster and more often) in a weird way and I couldn't find a solution yet. It started crashing when I had lots of programs open and played certain games (WoW, HoTs mainly). Now it happens on every game i play, even if its the only thing thats running.

The way it crashes isnt always the same as well:

-one monitor turns into one solid colour (either black, blue or some kind of beige?)

-the other one either stays normal and then crashes to a blackscreen or completely glitches or it goes straight to one of the two states

-either the pc crashes and I can only hard reset or it "fixes" itself (you can hear the sound like when installing a driver or plugging something in) and the current programs either crash or keep working

-I can almost always hear everything still working (even discord voice chats, but they cant hear me though)

What I tried so far:

-changing RAM

-changed PSU

-new thermal paste

-checking temps and using fan control

-up- and downgrading drivers

-switch to different PCI slot (from PCI x16 to x4)

-cleaned it from dust

-fresh windows install (from win8 to 10, this fixed it for a while but then the problems came back)

-had the same problem my previous GPU (GTX 960), which is now in a different PC and has no issues there

I got a GTX 980 TI (gifted from my brother with a PSU) and only an old motherboard (Gigabyte GA-H77-D3H) + i5-3470. Cant afford a whole new setup right now, but I could swap something out, if I knew where the problem is (I assume motherboard and/or CPU)