My Xbox 360 wont read my new games!!!!!! ...

i recently purchased saints row 2, fable 2 and fallout 3. my saints row 2 sometimes freezes right in the middle of my game and then says ''disc is unreadable''! my fable 2 did that for a while, then stopped, and now its doing it again. my fallout 3 (which is the one i really care about) always does that when i start my file. i have brought fallout 3 back to the shop for another one and it still doesnt work! all the games that i got in the past e.g halo 3, still work fine! can anyone tell me whats wrong with my xbox 360 and/or tell me how to fix it, because my warranty has expired!My Xbox 360 wont read my new games!!!!!! ...
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maybelline