Finding the Fault

Hi all. I am having a problem which I am struggling to trace. I am far from an audiophile, which my equipment will attest to, although I am an apprentice wanabee . I have a Sony amp and cd player and a pair of Quad 11L speakers. The amp and cd player are about 6 years old. The problem :- After powering on the seperates and playing a cd,certain parts of the music have severe distortion at all volume levels.This happens across a range of cd's. The cd never skips and never has any problems seeking etc.After trying the standard lens cleaning remedies it seemed that cleaning makes no difference, however, the problem would gradually reduce to almost negligable levels after around 15-20 minutes. I started to think that it was the amp which was starting to die, and this was the thing that needed "warming up" to work. So I turned the amp off and left it to cool down, turned it back on and tried the tape deck and turntable. Neither produced any distortion, but I then tried the cd player again and the distortion was back. Again it disappeared after I left it for a few minutes.
Has anyone seen this kind of thing before and if so do they have any ideas. It is the only cd player or amp I have so I can't swap out the separates to help me. I was planning on buying a new cd player (either a Nad or Arcam) in the near future, but I may need a new amp first.
Thanks