Flagging on top of screen only when used with agile modulator

[zenith2134]

I have determined the problem. Something I overlooked. It is the tapes themselves causing this. It is just more or less prominent depending on which player I use.
Some tapes have it bad and others don't have it at all. Some are used/second-hand and that is likely the reason.
What causes this anyway? None of my older tapes have it, it's mostly on the newer ones. And I assume nothing can be done about it?