How do you know it’s not overheating? Are you watching the temperatures with a program like HWInfo64?
It could be a software issue. Have you tried using Display Driver Uninstaller to eliminate the current Graphics card drivers, then reinstalling them. Have you tried making sure you have Windows up to date and all the drivers are installed?
If the PC cant pass a battery of stress and benchmark tests like Prime95 Blend, Aida64, Cinebench r15 and r20, Firestrike, Unigine Superposition, and Furmark then it’s most likely a hardware issue.
Have you tried running the game with only 1 stick if RAM?
The issue could be a bad power supply, bad graphics card, a bad stick of RAM, drivers or overheating.