I would say cause of glitchiness in all games graphics after say 15 minutes of playing is likely due to overheating have you checked temperature of the cpu etc whilst playing ?
when it reaches its max ceiling temp the computer would likely either shutdown or reboot on its own
Here i googled how to check cpu temperature and got https://www.pcworld.com/article/3253340/how-to-check-your-cpu-temperature.html
If the system is overheating you need to work out why and improve the cooling, could be a loose heatsink on the cpu to a fan not working for example
