Have you used a cleaning disc? I would strongly suggest not to use these on a DVD player. On a CD player they are not very effective to begin with, but on a DVD player they could really cause damage to the optical system.
Suggestion: try different discs. Reflectivity of discs varies and as your player gets older, it gets less "tolerant" of such variances.
Or, it could be that your player's laser has reached its end of life. Without taking measurements, you could expect this to happen nowadays once players get between 5 and 10 years of age (depending on how often you play discs).
It depends on what you paid for the DVD player. If it's a low-end unit, think of it as a toaster. You pug it in and either it works or it doesn't. When it breaks it's not worth more than perfunctory troublehsooting before you buy a new one.
If you have a high-end audiophyle unit, you may (should) be willing to do more to protect your investment. One thing you can do is clean the lens. Find yourself a non-abrasive cleaner that won't damage the lens. Although there many abrasive cleaning products sold, they should never be used on a lens. Cleaning the lens should be done at least every 100 to 200 hours of play or every month. If there are smokers in the house, the lens must be cleaned more frequently.