Pioneer CLD-D515 Picture Flashing

I think there is a problem with playback on my Pioneer CLD-D515. It was damaged in the post, which could be the reason.

During playback, the picture "flashes" constantly, it is particularly noticeable on black or very dark areas of the screen which flicker between the normal colour and a very light shade.

The problem is more pronounced on some discs than others, but it is definitely a problem with this player as the same discs play fine on my other players.

Would anyone know what is causing the problem and how it can be repaired?