[Formulajones]

As Steve said I'd be curious on valve spring pressures too.
Since it was built 20 years ago can't necessarily blame it on faulty parts, a very slim chance anyway. Only 4000 miles tells me it sits a bunch so a lot of extended sitting cold starts comes to mind. Not that that's the problem but not the best thing for it either.

Probably the why isn't all that important to you now, just gotta cover your bases 2nd time around.