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).
