Flagging on top of screen only when used with agile modulator

[zenith2134]

I think I'll simply invest in a timebase corrector. The only tv set I will look into modifying is my 1979 Zenith system 3 25", because I run an old VTR right into it, rather than over the air with an agile modulator. I have also seen professional VHS decks with a skewing control..There's a chance that adjusting the tape tension that way could also improve the flagging issue.