I suspect that your copy of FFmpeg is also re-encoding using the defaults for the avi container, which can be a little unforgiving. Try running the following simple additions to your commandline:
ffmpeg -i in.avi \
-filter:v "setpts=2.0*PTS" \
-c:v mpeg4 -q:v 2 \
The driving part of this commandline is the 'quality' setting:
-q:v 2 which can be set from 1-31 with the highest quality being 1 and the lowest being 31. Note as well that I have used
-an to bar use of the audio stream.
If this effectively increases the quality of your output video you could consider adding the following flags to your video line:
-vtag XVID -f avi -mbd rd -flags +mv4+aic -trellis 2 -cmp 2 -subcmp 2 -g 300
I routinely use these flags when encoding for an older device and they do produce a significantly better result...