Hi, and welcome.
The 5902 is actually one of the worst JVC unit, being a latter-generation 4600-4900, but with even cheaper parts inside. It was good for recording, but not playback. I used 3800s for years for recording, but palyback quality was blah. Better than a normal VHS VCR, yes, but not by much compared to JVC 7600+/9600+ units (or SR-V10/101).
It's not worth fixing if it has serious errors.
However, it sounds like typical JVC aging of the alignment. Every 2-3 year, most JVC decks a misaligned due to gravity. The positioning system isn't as exact as others. (This is a good thing, however, as it means you can purposely misalign JVC decks! Very useful for advanced restoration work.)
I've been using these decks for 15+ years now. Seen this over and over and over. Easy fix.
The machine auto-tracks the tape while it plays (I don't even know if this thing will let me manually track!)...that has been a strong point of the machine up to this point. But lately, it seems like it's constantly tracking, and not accurately. I can put a pre-recorded VHS tape in that I know is in good shape, and it seems like it spends the entire duration of the tape moving briefly into the "sweet spot", moving out of the "sweet spot" until the picture's riddled with tracking lines/artifacts, then back into the "sweet spot". It didn't used to do this.
