Playing DVDs

Two possibilities: Bad disc or bad player. If it does it on ALL discs, even ones you know are good, unscratched, etc... then look into replacing (or having applecare) replace your drive.

If you are out of your warranty and don't want to spend the money to replace it, you can always get an external drive to play discs, or attempt the replacement yourself - Depending on the model it can be easy as cake to something that should be left to the pros.