@jitters most common cause for that is built up of dirt and grime on the laser assembly. This will of course only hold true if you made sure that your DVD's are not scratched, dirty etc. Before anything else you can try to clean the DVD player with a commercially available lens cleaner and/or a few puffs of compresses in a can . Otherwise you have to try to disassemble your unit and clean the lens or start thinking about replacing the laser assembly.