This is due to the source being interlaced, and hence that interlacing being encoded into the digital version. I was able to use http://handbrake.fr/ on a local test version to remove the interlacing and this cleared up the problem.
Are you able to export a version without interlacing?
In PR9 i'm using Share / Computer / MPEG / HD 720p 25, which works fine in all other respects, the problem doesn't show itself if I export as PAL DVD widescreen, but obviously that's not HD.
I don't see any interlacing options, so am I going to have to Handbrake on all my videos?
Cheers,
Bernie