only 2 things can be here :
1. Your DVD is scrached,dirty or damaged somehow
2.your DVD player lens is dirty..
In case of 1st problem u acn try to clean DVD with soft cloth and if there is a scratch u can try to polish that by gently moving and rubbing from iner to outer part of the disc...
In case of 2nd thing u can try to clean your DVD by using DVD clean complet:inserting special dvd and starting small software that will clean your optics...
if this does not helping then......
