Scrolling bars using agile modulator

[bigaudioal]

I have noticed that recently both my working vintage sets, when using my Blonder Tongue agile modulator, have scrolling bars moving up the screen. This morning I was watching Bob's latest YouTube video and whatta ya know . . . he mentions this is sometimes an issue when using these units or it could be a video source issue as well. Has anyone identified what these bars are caused by? Can see them in Bob's below video.

https://www.youtube.com/watch?v=_oNk...em-subs_digest

He mentions it at the 22:20 mark for a few minutes.

I have been using this unit for the last year and it has only started within the past few weeks. Has anyone identified the issue and figured out how to fix it? If anyone has diagnosed and fixed this I would love to hear about it or be directed to any old forum posts that talk about these bars and what causes it.

Thanks!!!