Thanks for the reply.
The CD I've been using for checking gives identical results in both Windows Media Player and iTunes.
The original files, which I recorded onto the CD, are still on Hard Disk; they play perfectly in either program.
The CD also plays perfectly, in either program, using my DVD Recorder.
Other Audio CDs also give the same problem on the PlexWriter.
I've cleaned the lens (Isopropyl alcohol) at least half a dozen times over the last week and, as stated before, one track did play OK before failing again.