I located what I am sure was the original mp3 I ripped of the song in my collection on my PC and tested it in Winamp and it sounds fine - no distortion or anything.

I haven't actually downloaded the faulty track from the player and tested to be 100% certain its the same track but the file size agrees and I have only 1 copy of the track on my PC - but I think I'll try the download it and test in winamp as well that just to be sure in case the upload process glitched the file but it sounds too much like a playback/mp3 engine decoder bug to me.