Interference on some copied CD's when playing in Dance unit

I'm experiencing some interference on some 'copied for the car' cd discs when playing in my 'Dance' radio/cd player. I copied some of my originals in order to preserve them from damage in the car. The original discs play ok but the one's I copied get to track 6 then develop some nasty bursts of clicking and static. It gets worse as the disc continues to play. Having mentioned it to my Skoda dealer he tells me it can be a problem with using cheap cd discs, I'm using SONY. However, it could be a fault starting in the head unit itself. My dealer was happy enough to replace the head unit under warranty. This has been done, for which I'm grateful. However, whilst there has been a distinct improvement with some burnt discs playing fine now, some still give me interference.

Anyone else experienced this? If so anyone know why? I guess I ought to make more use of my MP3 player as this performs faultlessly through the Dance unit. I would add that whilst I know lots about engines and gearboxes, cd players are a mystery to me, along with women (don't tell my wife), politics and religion!