NAPA MP3 (DAV311) problems.

Hi, I've got one of these Portable CD/VCD/MP3/CD-RW players.
Playing music CD/CD-RW disks and VCD disks seem to work fine apart from the
occasional error in decoding the mpeg video but MP3 playback is total crap.
This is rather puzzling since VCD audio is flawless and mp2 audio isn't
much different to mp3 audio.
Basically when playing mp3s the sound quality is so bad that it's not
worth listening too. It seems to have a good frequency range but suffers
from what sounds like nasty harmonic distortion.
Are there any NAPA DAV311 owners who have the same problem ?
(note I'm not interested in different NAPA models only the DAV311)

Richard S.