Mitsubishi HS-U595 erases a scanline or two during play?
 

I have a Mitsubishi HS-U595 with a very curious issue: When playing a tape, it seems to erase a very thin horizontal part of the tape, maybe only a scanline or two high, about 20% of the way down from the top. The very first time it plays a section of tape, playback is perfect, but if I rewind and replay that segment -- or if I pull the tape without rewinding, then rewind in a different unit and play it there -- it's obvious that a portion of the video has been erased: there's a black line horizontally across the entire image with slightly rough edges. The tape is *not* damaged; not only do I not see any physical damage at all, but I can use any other VCR to rerecord that segment of tape, then play it back, and it's fine again. It's almost like the VCR is degaussing or rerecording, but only over a tiny portion of the whole video field or frame. The heads look fine; I also very carefully cleaned them and it made zero difference, but I'd think head damage or dirt would affect the *entire* path taken by that head across the tape, and a single scanline or two is obviously only a tiny part of that path.

Anyone have any ideas? I mean, this *could* be an electronic problem in which one head is spontaneously turning on its write amp during part of the scan, but that would be pretty weird.

This unit has a fine transport, so if I really can't figure it out, I may swap its transport with a unit that isn't displaying this problem but whose transport is marginal (see my immediately prior message re possible belt issues and sizes thereof), hence making one perfect unit out of two bad ones, but I'd rather not do that if there's some simpler explanation. Thanks!

P.S. I don't have a screencap of the issue, but if nobody can figure it out without that, I can probably arrange to either find an already-damaged tape section, or damage another one to do the capture.