All you can do: use a cd cleaner. If that doesn't help: replace the unit.
Most modern players will play copied cd's; some won't. Be sure you buy the right one (so you can return it if it doesn't)/
I have a Volvo C70 and the car stereo which is a 3 disc changer has never played copied cd's (which is annoying enough) but now it only plays 40% of real cd's.
Is there anything i could do??? I have heard about stereo unlock codes and burning blank cd's a certain way to trick the stereo but im unsure?