Xbox 360 unreadable disk error?

it happens while i am playing cod world at war, and gta 4. and it rarely happens with all my other games, but still happens from time to time. i do not think my cod cd is scratched and i tried tons of cleaning methods. but it works pefectly on my freinds xbox so i think it is the xbox not my game.i tried installing the game to my hard drive, but it still happens. it normally happens when a game ends and the loading screen comes up. the worst part is i got this xbox about a week ago after waiting for mine to get repaired for about a month.