I noticed that on my recent avi to kvcd conversions, I get
artifacts/distortions in the beginning of the movie and in the end when the
credits are rolling.
Also, because I usually join 2 avi files (when the movie is split in 2 files), I
see artifacts/distortions right where the second part begins.
But otherwise, the picture is good.
I use 528x480, but 480x480 also had bad artifacts.
I use min birate of 300 and max 2500 (the default values).
What could be the problem? (I know that source had no problems.)

Which tool do you use for the join ? You do not need to join them in fact, as avisynth can do that itself.

I used virtual dub.
How do you join 2 avi files with avisynth?
I have never seen an example.

So simple.

Great! I'll try that next time.
Thanks Dialhot.