Does it use Nvidia Optimus? If it does often the signal from the Nvidia GPU is sent through the integrated GPU, so disabling will cause what you saw. Also it has a tendency, at least in my experience to switch GPU's under light load and generally be a pain in the butt. I think settings pertaining to Optimus are in the Nvidia Control Panel? I cannae remember as it's been a few years since I had a laptop with it but I recall there was an function key press and a reboot required. Not sure if it's yer issue but jus food fer thought!