What was the purpose of the line that ran through the middle of the image of
Spacelab transmissions? It was a black line with smaller white lines moving
horizontally.

I only saw it in Spacelab transmissions, so it wasn't my TV. I saw it on
every flight. If it were data, it was a poor decision, since the vertical
blanking interval or, for that matter, the top or bottom of the screen would
have made more sense.