"Peter R." wrote ...
.... there are about 180 hours or so on this air-driven AI ....

... the AI, which normally rests in almost level mode and spins
right up at avionics-on, ...

I'm confused by your details. You refer twice to expecting action at
Avionics Master On, but claim the AI is vac sourced???
Are you talking about your Vac driven AI (on with engine rpm) or your
electric backup AI (on with master, I've only seen these wired thru a CB,
never the avionics master)??
And the most important question: What is your vac gauge indicating?