Gremlins with transcoding

Having successfully created a DVD for part one of a wedding vide project, I have run into problems transcoding the timeline from HD to SD. I've followed the same procedures as per usual and copied and pasted to SD timeline (having changed the settings). The first 4 and last 15 minutes are fine - the middle bit is stretched and zoomed in. I can't think why. The only things I've done different is to have used more video and audio tracks than per usual and changed my setting on the video processing from 8 to floating 10 bit. What I want to avoid is re-editing the distorted part of the sequence. I've tried transcoding a number of times all with the same outcome. ANy one out there got any bright ideas? Your help would be most appreciated.