I've seen a lot of things, but never this.
Acquired a non-working SR-V101 deck in trade, good refurb candidate. Had gunk on lower stationary head cylinder. Cleaned that off. Some transport ingest/eject issues, also caused by grime and bad lube. Alignment was shot, way off, wouldn't even lock SP. Adjusted guides, better now. An "easy" job, but time-consuming as usual, took many hours.
The unit meets my A+ criteria for tracking EP mode VHS tapes. Auto-tracking is spot-on almost every time now. The image is clean, audio clear. This is actually an unusually nice JVC deck! So the heads appear to be fine -- better than fine.
But on S-VHS and S-VHS-ET tapes, it suffers from dropout comets on screen (tracking based?). Then tracking creep, both sides randomly. Then loss of v-hold. Repeat.
The VHS is perfect.
How could only S-VHS be affected?