separatefields() and qtgmc() cause a 'jump' at the bottom of the video

Hi. As I usually do with a DVD, I scripted it with megui and checked for combing. There was combing and so I added separatefields() to the script to confirm it was interlaced (it was). The video preview, however, is now 'jumping' as if every second frame is a different height than its neighbour by a pixel. I added assumetff() then assumebff() prior to separatefields() but to no effect. Scripting with just qtgmc() also causes the jump.

Hopefully someone recognises what I'm describing and can tell me the cause and cure. Thank you.