If it was a black and white picture, it could be there were some
artifacts caused by the film to video conversion. Did you check any
other sets or neighbors' tvs to see if they saw the same thing.

I had the same effect with two different brands of convertor boxes and on
different TV's. My antenna was common to both. I don't know if too much
signal could have caused it, or if it was broadcast with a problem. Most of
the local broadcasters did seem to need a month or two to get things right,
such as the sync between the video and audio, going off air etc ...
and I was tweaking the antenna quite often too, wondering if it was me or
them.