Have you cleaned it?
DVDs out of the pack commonly have marks...etc on them which affect playback (and this is before you get your own fingerprints on it or drop it on the floor and get dust)
Use a fluid cleaner and a proper cleaning cloth (one of those rotating cleaners is sufficient, same stuff used for CDs) and make sure it's totally dry before putting it back into the player.
Using a soft, dry cloth gets rid of visible dust, but it isn't any good if other marks are present.
I've had quite unplayable DVDs from the local hire place play quite nicely once they have been given a thorough, but careful clean.