ffmpeg re-scaling results in a ripple effect

I'm re-encoding and resizing a few videos from .mpg to .mp4 using ffmpeg, from 704x576 to 640x524Code:-vf "scale=640:-1"this results in a rippling effect on any rapid movement in that part of the video.

Removing the -vf scale switch and the mp4 re-encoded file is fine.
Anyone have a few pointers as to what I should look for, ddg hasn't uncovered any clues.