Craig and Paul,
Thanks to both of you for info and suggestions, especially your offers to swap boards for testing.
I've got an HP LogicDart that makes it pretty easy to look around for pulsed input, so I'll use that tomorrow to try to find any activity on A25 that indicates the Monitor Bus cable is intact. I haven't yet actually pulled the processor assembly out of the case, so I'm still hoping that there may be a loose/bad/dirty connection that will be easy to resolve but I realize that may be unrealistic.
I'll be able to put in a couple of hours tomorrow morning but then will be away for a couple of days. I'll post an update in the morning.
I have a scope and an older HP logic analyzer so I can also dig deeper if necessary, but without timing diagrams or a comparison board, I'm not too sure how far I'll be able to go.