What is happening to my Xbox 360?

Whenever me or my brother play games on our xbox 360 it will say that certain discs are unreadable. This has only happened to some games like the new prince of persia game, my brother's NBA game and just recently the first modern warfare game. it will happen randomly to games as shown by the modern warfare since it worked fine the day before and then wouldn't work. This not only has happened on the original Xbox 360 but also the Xbox 360 elite we just got for Christmas to replace. Any ideas on what might be happening and how to fix it would be appreciated.