It looks like head switching noise to me, although normally you would only see it affect ~10 lines at the bottom of the capture. Makes me think that this tape is not first generation, but a "copy of a copy". Have you tried other tapes, maybe better quality retail originals, and do you get the same noise?

I would hazard a guess that the reason you don't see this on your TV is that it occurs in the overscan region of the picture.

The solution is to crop the top of the picture, either during capture if your software allows it and your system is fast enough, or during post-processing.