I would try cleaning the disk first. Do you see scratches on it?
Don't DVD players need cleaning sometimes if they have played too many dirty DVDs with fingerprints or dust on them?
There is a lens inside the player, which is like the "pick-up or sensor" for the laser, that can become cloudy with dust. It should probably be cleaned by a professional, though, since the machine will have to be taken apart. It is a little dangerous to take them apart yourself, because they are actually assembled by robots, using jigs to perfectly align the cabinet pieces. Most of the cheaper ones are made this way and once you take them apart, the doors don't line up anymore or the internal mechanism shift and they will not work well anymore.
The thing is - if your machine is a couple of years old, you can buy a better machine for fifty bucks or an upgrade to a much better machine for around a hundred, these days. It will cost nearly a hundred for a cleaning at most places!