Thanks for the ideas. I just removed and cleaned the wire connections at the starter and at the back of the ignition switch. The fusebox is out of the ignition system entirely. No funny corrosion found - although I sandwiched a couple copper washers in the solenoid connection to keep all the wires nice and "juicy" when tightened down. Maybe a bad idea, but I don't see an obvious risk.

I ohm-metered the ignition wires all checked out with 0 ohms. I also and re-insulated the ends of the ignition cables.

I discovered (with a slip of my multimeter probe) that the car will crank just fine (as it has been doing when it has stalled) but will not fire if either the key is off or the wire to the coil is disconnected.

To me this suggests the wire from the armored cable is where the problem lies.

I still can't quite figure out the intermittent nature of the problem and have been unable to reproduce the failure so far - making this hard to properly diagnose...