It seems like it might be a blown preamp transistor. Some rules of thumb...does the distortion happen in one channel, or after the system heats up? Does this occur only during CD, or does it happen on the FM band as well? THat might give you a clue where to look.Our high school electronics teacher always cussed "those damn japanese self-healing transistors". Try some liquid freon or tuner spray to see if you can isolate the location by spraying a cooling jet on the problem area. If you take a small cheapy speaker and carefully poke around, you might be able to trace the signal to the offending part. I found that it is usually easier to just replace the final amps (those large honkers usually heatsinked to the chassis AND preamps together since it seems to always sound different in each channel unless you treat them the same. If you can get yourself a schematic (SAMS Photofacts) that can be of great help too.
BTW, a cheapy new one is usually much less hassle (you have to pull it out and remount it when your done anyways...) unless you are in for the challenge of it. Just be SURE you have solved the problem on the bench before you go thru the hassle of reinstalling only to discover that that intermittant problem is STILL haunting you.
Best of luck
Good luck
Ian, June 2006