There are a couple of things that spring to mind, depending on how the problem manifests itself. Is it always the same place on the CDs each time you play them, or does it vary from time to time?
The first thing I would try is to unplug the connecting wires (while the power is off) and replug them in to see if you just have a bad connection, as sometimes (rarely) mild corrosion can occur on connections and give the kind of problem you mention. It is also possible that the laser is going on your changer and cannot read the CDs as well as the single disc player (or maybe its error correction was always inferior to your particular single disc player).
