Thanks guys, RRobert, Rhett, Scott and all..
Just coming back to report the results

- I fiddled with the plugs on the back of the ignition switch, something didn't feel quite right there, but I couldn't actually see everything. I pulled the plugs off, took the ignition switch out, it looked clean, put it back in, then re-seated the plugs.

- I pulled the firewall plugs and I think that's where the problem really was: I blew a lot of dust out of it - as if there may have been some arching going on in there - I didn't see any real corrosion, blew the plugs and sockets clean with compressed air, greased them and replugged them several times before seating them with the clips..

Drove the car, several days now, no sign of any trouble.
Cheers!
- Art

PS - Oh. and I scoped out the ammeter connections and they were solid.