[old_tv_nut]

One field per swipe.
If it's on the tape, it would be part way between the top edge and the bottom, edge, proportional to the vertical position in the picture.

Tape damage would also show up in both fields, but the examples you posted seem to be in alternate fields.

I think this is too minor to be tape damage as such. Damaged tapes typically produce a much wider noise stripe and really wreck the picture as the wrinkle passes by, and produce a buzzing sound as the heads run over the wrinkle.

I still can't come up with an idea of what's going on.

Grasping at straws: Is there a stationary part of the drum that you can get to and ground?