I have a Nvidia GTX 690. When I am using 2 screens the preview window goes all crazy unless I have an overlay (either grid or safe areas) on the window, then it operates just fine. If I am using one monitor, with or without GPU processing turned on, all is fine.
If you have 2 screens, try turning one of them off just to render your project and see if that doesn’t work for you.
While it sounds weird, well, it really is.
Royal