After some troubleshooting - some of which was varying the length and contents of the ID3 tag fields, and some of which was playing around with MP3trim (Thanks Tony!) - it looks pretty much like parts of the files were corrupted. All 3 showed a missing first frame, and an incomplete last frame. After fixing this with mp3trim, everything worked fine.

Kind of puzzled about how this happened, though...

Bill B.
Mk.2 SN 080000183 - 18 GB / Green