CoD Blk Ops won't let me load Multiplayer!?

I recently bought an Xbox 360 from a friend of mine...It's in pretty good condition. But I'm borrowing Black Ops from another friend of mine and it wont let me play Multiplayer. I got Xbox Live last Friday and every time I click Multiplayer it says the disc is unreadable and takes me back to my Dashboard. It'll work every once and a while but RARELY....Does ANYONE know wtf is wrong!