How about Filters-> field swap ?
If that screws things up, you could try Donald Graft's Reverse Field Dominance filter. (Put it in \VirtualDub\plugins).
And if that ain't it, a screenshot is worth at least 50 words... better yet, a screenshot with the downconvert filter, and the same frame without.
What's camera model, and what codec is it giving you? Maybe we could dig up something to parse the file. Have you used this HD camera for all your projects? If it's a different one, is the codec different? Maybe your resize filter, or sharpening filter hate the codec, or resolution, or somethng. Do you know if it's true 30p, or if the camera is playing some tricks, so that marketing can label it 30p? How did you decode the camera's codec into lagarith? Are you using Vdub or VdubMod, or...
Just spitballing, in case I catch your next post late, again. :)