The older systems have a hard time running the game in official non the less singleplayer. In single player the load of the server is on your xbox, which can cause overheating, and overall damage to your system.
Unfortunately, support for this older xboxs lessens every day for all games across the board, so I suggest upgrading when avaliable . You wont regret it.
Secondly, if you enter the options menu from the main menu, you have access to the graphics options and such. Switch to performance , save then restart your game.