I tried using each of the three interlacing options that could be chosen in the MPEG capture setup.

I don't know if the MPEG capture is a separate set of options from the regular video card settings on your card. But you said you also noticed the issue when simply viewing video images. So are there other options somewhere else in the video card settings, somewhere other than the MPEG capture settings?

Beyond that, I'd start looking for hardware/physical problems, like a bad video cable or a ground loop between your PC and your video source.