Using the open source driver with Nvidia is a bad idea, your card is locked at the minimum clock speed and it's general quality is not comparable to the proprietary driver (this is purely because of Nvidia's hostility to open source, not due to any inabilities of the developers of Nouvea.)
I'm gonna assume you are using the default desktop environment of Mint which is Cinnamon. Have you tried booting a different DE, or even better, a different distribution with something like Gnome or KDE to see if the issue persists?