FPS drop after Alt+Tab and weird black squares

So some of my friends and myself have experienced random frame drops down to the low 40/30 fps. The drops occure sometimes when we Alt+Tab from the game itself. We also start the game with around 112-130 fps instead of the usual 200-300 fps. And no, this is not our cards, since we have tested them in a variety of benchmarks and games to check if that was so. We have to DC/RC from the game just to get the fps back to "normal" again.

The GPU's that this is happening on is at least the GTX 1070 and GTX 1080Ti And the CPU's are a Ryzen 1700 and a Intel Core i7 4790K

There has also been alot of black squares appearing randomly across the screen.

Have tried changing visual settings. Didn't work Have tried running borderless/windowed. Didn't work Have tried to reinstall the game. Didn't work Have tried to reinstall the drivers. Didn't work Did try to swap GPU's, still didn't work. Did not try to swap the CPU's though.

Anyone having the same issues? Or anyone who knows a fix for this?

Good day ^^