Line intermittently cutting through image?

[lolachrome]

Hi there--I'm new to this community, and I could really do with some help! I am finishing up a vid that's been quite a cause of technical frustration, because I decided to use DV rather than avi, possibly... I've resolved most of the issues I was having, but am left with just one. When I play the vid in FCP, it looks fine. But when I export to either divx or to mov and then to mpeg4, it ends up with this intermittent white line bisecting the image. Anyone seen this before and know what it is? It's not the interlacing crosshatch--it's a long line that cuts right through the image, as if a razor had sliced it in two.

Any suggestions would be very, very welcome! Thanks!