After each "instance" in OP, the car was completely dead - no lights, horn, etc.
Diagnosed that we were not getting 12V at the BATT terminal on ignition switch, and:
- Battery is fine (12.8v and load tested)
- Continuity through master disconnect is OK
- Continuity through fusible link is OK
- Ignition switch is ok
Circuit then goes through ammeter and then to BATT terminal.
So then we ran a jumper lead from battery to BATT terminal on ignition switch. Car was then energized, started, and runs fine.
Could the ammeter "stick" and create and open circuit ? Could energizing the other side of the ammeter "unstick" it and resume a continuous circuit ?
Somehow an open occurred between circuits 25 and 27 in above diagram - that seems to rectify itself by jumping to ignition switch ?
Don't see how it could be the alternator or voltage regulator - they are not part of the circuit in between the ignition switch and battery.