game window keeps glitching

Sahugani

so I've had this issue before but after the last update or so its gotten worse. when I press ctrl to crouch the video glitches and flickers and after a few minutes it will suddenly drop into windowed mode except if I go to my settings it shows that I'm still in full screen. if I switch it to windowed it fixes it for a minute but eventually returns to windowed if I try to stealth again. the only thing that really fixes it for any significant period of time is quitting the game and reloading. this only happens when I crouch to stealth nothing else seems to cause this.