I recently aquired an immaculate CD3500, which works in every respect as is should, some of the time. Some CDs it will scan and display the tracks, then switch off, some it won't scan at all, a few it will play but with massive distortion, and others it will play perfectly. The CDs it doesn't like don't seem to be any more or less damaged than the ones it plays. Is it just dirty, or is it more serious? Thanks for any advice