Interlace looks fine on preview, awful on export

Interlacing has always been rather confounding. Here is what I've got: Original NTSC VHS footage dubbed onto DV tape, then captured into FCPX. The footage looks fine in the browser, fine in the timeline, fine in the preview window. But a Master File export yields combed images. My understanding is that a Master File export is WYSIWYG. But does the preview window so some on the fly interlacing/deinterlacing? The reason I wonder is that if I change the project from 480i to 480p it makes no difference in the preview or the export. Preview looks great in both cases and export is combed in both cases. Where/when is additional processing happening that does not show up in preview? And how do I get the export to match what I'm seeing? Thanks for any help.

Set the viewer appearance to show both fields.

Can you post a screenshot of your interlaced export file?