Yep, pushing your GPU too hardand it might be time for good cleaning. Keep in mind the GTX 750 is now several generations old and cant run any game thats come out in the last couple of years at anything past low settings. Here is a bench comparison between the GTX750 and 1080TI. Yes big spread but wont be loing before 1080ti will be a thing of the past.
http://gpu.userbenchmark.com/Compare/Nvidia-GTX-750-vs-Nvidia-GTX-1080-Ti/3162vs3918Those cases are tiny and tight. I considered one awhile back just for the novelty then swiftly decided against it most due to limited upgrades and the fact that there were laptops being produced with better specs. They are prone to overheating as there isn't much air flow, just add a bit of time to the equation for dust accumulation and everything gets hot. 3-4 years in the realm of computing technology is an eternity. Im sure your machine works fine for most things.
For the most part this series machine is a laptop without a display turned in its side. BIOS settings are extremely limited and with good reason. You only have 180Watts of power to deal with. Just for a comparison my latest build consumes 5 times that under load. One 1080 Ti pulls nearly double that of your entire machine.
So quick and dirty......take off as many covers as you can and blow it out with a compressor, not compressed air in a can (pretty much worthless for this task). Dont go crazy with a milllion PSI, about 80 is fine. Once you get as much of the crud out as you can then turn down the game settings until you can keep it stable. Dont expect much as the intel Iris on die GPU falls only about 30% short of the GTX750.
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein