erratic

That's because some DVDs are interlaced. If an NTSC movie is interlaced
(framerate 29.97 fps) you normally use IVTC (inverse telecine) to turn
it into a 23.976 fps progressive AVI file. Your DVD ripping program
should have a setting for it, I suppose. I'm in PAL-land where IVTC
doesn't exist so I can't offer more help, but you can find a lot of
information at http://www.doom9.org or in the forum there. Read this for
starters: http://forum.doom9.org/showthread.php?&threadid=50654

arsenio

Select de-interlacing on your encoding prog.

ARSENIO

erratic

Yes, for PAL deinterlacing is sufficient, but for NTSC inverse telecine
(IVTC) is generally much better. But it depends on the source. If it's
truly interlaced video (like a TV show) deinterlacing is better than
IVTC. If it's a telecined movie (that's an NTSC only thing, I believe)
IVTC is the best solution.

erratic

Apparently it's not NTSC only.
http://forum.doom9.org/showthread.php?&threadid=28066
A few quotes from that thread:

"I have a PAL interlaced source that looks rather strange to me.
Playing it back frame by frame there are 13 frames that look normal
(they don't look interlaced) then there are 12 frames that are
interlaced. It then appears to repeat this process over and over."

"This DVD has probably been 'PAL telecined' (not a good name but that's
exactly what is going on), i.e. one half frame has been added every 12
full frames!"

But never mind, I don't rip DVDs so it doesn't affect me. :-)