Latest results. I transcoded the same file with Konttori's MediaConverter, setting Xvid as the codec. The file played jerkily from almost the beginning in mplayer, increasingly dropping frames and soundfragments as it went along.

The same file, transcoded with PocketDivXEncoder and the same settings (or so I thought, see below) played without any major flaws at all (there is a very slight, extremely short stutter every three minutes or so, but this has been the case with the tablets from the beginning; it used to be much, much worse).

This is the file information mplayer on the tablet gives:
For the MediaConverter file:
video codec: xvid
bitrate: 499 kbps
res: 400x224
audio codec: mad
bitrate: 32 kbps
samples: 48000 hz 2 ch

For the PocketDivXEncoder file:
video codec: xvid
bitrate: 483 kbps
res: 400x224
audio codec: mad
bitrate: 96 kbps
samples: 48000 hz 2 ch

They look remarkably similar, save two problems:

• Both MediaConverter and PDE claimed they had the audio bitrate set at 128 kbps, while mplayer reports them at 32 and 96 kbps respectively;
• the video bitrates were supposed to be 512 kbps according to the transcoders, but again mplayer shows a difference.

I don't know whether this discrepancy is mplayer's "fault" or that the transcoders are off, but it is telling that the file with the lowest audio bitrate is also the one causing playback troubles.