[old_tv_nut]

It's hard to tell from the photos if the lines are closer spaced in the light area or not. Did you look for distorition in a picture?

Is this effect as strong with a picture as with a plain raster like this?

If both vertical output tubes are conduction during the whole waveform as I think they are, it makes it more likely that this is some sort of glitch in the video drive rather than the vertical.