The artifact you're getting is probably not due to the temperature although I cannot be too sure but I personally never had artifacting because of high temperature and I had my GPU reach 95°C.
Hopefully its because of software issuse and not hardware.
Try to remove current nvidia drivers and install latest ones or older version.
Remove then with Display Driver Uninstaller.
The GPU temperatures will decrease if you repaste the laptop and clean it. I recommend a good thermal compound like latest noctua paste or something similar.
Asus Rog G750JZ
Screen: 1920x1080 17.3'
CPU: Intel® Core™ i7-4700HQ (3.4Ghz @ -70mV)
GPU: GeForce GTX 880M 4GB (OC +125Mhz Core, +600Mhz Memory @ 1.062V)
RAM: Samsung 32GB 1600MHz
Storage #1: Samsung Evo 840 250GB
Storage #2: Samsung Evo 850 500GB
Storage #3: Seagate Barracuda 2TB
Storage #4: Samsung Evo 128GB MicroSD