Following a clean install of XP, I've reloaded Nero from it's original retail disk, and applied all the latest updates from the Ahead web-site (having first verified them using their MD5 checksums)
I now notice that the current version of Nero Showtime (220.127.116.11) doesn't support playback of MPGs or DVDs that contain PCM audio tracks. (These MPGs were created using TMPGEnc Xpress)
The effect is identical to using Windows Media Player without a DVD plugin: the audio is choppy, and the video judders badly.
From the Ahead site, I see that PCM support in Showtime was "fixed" some time ago:
But it seems to me that it's come back again.
I've done some experiementation and found that the playback of MPGs & DVDs containing MP2 or AC3 soundtracks is OK - as is PCM MPGs if played using Windows Media Player.
Sadly, to date, I've had nothing useful from Nero tech support.
Even if I can't resolve this, I'd be happier knowing that others have found the same problem...