Graphics glitching in games

Picked up a new rig about two months ago. I'm running an NVIDIA card with 6GB of VRAM. Games like GTA V or Mafia III run fine at 60 FPS on High or Ultra settings. But there's a catch: if I play for more than two or three hours straight, the game crashes. When I relaunch, everything has defaulted to the lowest possible settings. It looks like something out of the 90s. If I manually fix the settings and restart the game again, it works normally for a bit.

Why does my graphics performance keep tanking mid-session?🤔

First off, just lay out what components you're working with and let me know if the game is an original or some kind of knockoff