Hello unless someone better than i can help you all i can say is im suffering from the same problem and it can be many things. I just turn off the computer and restart it. When im playing large games i just have to keep saving it every so often. Usually like you said it could be the graphic card overheating but that usually blacks the screen on a regular basis and i use a small desktop fan next to the ventilator to cool it down, it does work quite well. Otherwise use a maleware software or spyware and scan your computer in case theres anything affecting it. Also check your graphic card drivers especially if you are playing those sort of games. Otherwise its coming to the end. Also use Auslogic defrag your files, Piriform CCleaner. All those things might help. Thats all i can do to help unless someone else has anymore ideas.
OnePlus 5 review