Its kind of hard to troubleshoot without more details or having the PC in person.
But, I can try some things.
As valleycat1 said, it could be overheating. Have you checked your CPU and graphics card temperatures? You can use something like EVGA PrecisonX, MSI Afterburner, CPU-z, and so on. If your computer is overheating, you might need to add more fans or install a better CPU cooler (bigger air-cooler or a liquid-cooler). You might also need to adjust the fan curve settings in the UEFI of your motherboard.
Next, make sure your computer is up-to-date. That includes: drivers, Windows, games, game clients (Steam, Origin, U-Play), and other software.