Followed this guide exactly https://www.videohelp.com/forum/userguides/186739.php, converting NTSC to m2v, having stripped out the sound. Usually works OK but this has happened with a couple of movies only. Judders very badly when tested in Xvid after encoding. Original avi plays OK.
Tried changing field order to Top Field first, but no change.

details from Gspot: Xvid avi. AC3 5.1. Stream type: OpenDML avi. Bitrate 1665kbps. FPS 23.97. 736x400

thanks all