Game stops working after a few minutes/can't save

[Kcowboy]

After playing a mission for a few minutes, the textures get blurry and people become invisible, I can't shoot my gun, can't get on my horse etc. I press escape to save and restart, and the game pauses and the black and white background comes up, but there are no options for me to press. I end up having to close the game without saving and my progress is lost. The game was working fine a few days ago and I can't think of anything I did that might cause this. Any help is appreciated. Screenshots