I had a 3-head Nak deck with exactly the same problem. It wasn't a
pinch roller problem...there was a flat leaf spring in the transport
that kept the cassette shell correctly positioned, and when it
snapped the tape wobbled around in much the same way as you describe.
I did buy a replacement spring, but gave up when I realized I'd have
to remove (and correctly reinstall and realign) the entire transport
to replace it.
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