Sorry to provide a negative reply, but "all" DVD equipments are notorious for these type of problems and if the discs affected are known to play on another machine and you have tried cleaning the laser more than once then it points to the laser having lost its efficiency, DVD/CD lasers being about the most erratic devices ever to be invented and with the price of the equipment they are fitted to having absolutely no bearing on their reliability ratings. (software problems can sometime cause, or add to this sort of problem, but I am unaware if any software updates are available for this model and its very doubtful if it would apply in your case anyway)