Flagging on top of screen only when used with agile modulator

[zenith2134]

I am having a strange issue with some equipment. I run three Blonder Tongue agile modulators in my place. They all have both DVD and VHS players used as their input. On one of these channels, however, the VHS tapes play with flagging at the top of the screen. I figured it was a tracking issue, but it wasn't . Next, I swapped out the VCR with two other known-good ones, and the problem persisted.
Now, I am aware that these B-T modulators can develop hum-bars due to poor power supply filtering action; but this issue is a new one on me. On the other rigs, all my VHS play perfectly. Yes I have also tried a number of TV receivers to eliminate that as the source of the problem. Any thoughts ?