78 starts, doesn't run

Was cruising along fine the other day, had noticed some sluggishness, and then engine just stops while at speed. Starts right away - cranks fine, sounds like it will run, but it doesn't when I release the key. No bad fuses found. This happened once before (at least I was in the driveway then) and I found a wire had fallen off a relay on the upper right fender in the engine compartment. Put it back on and was good. That was months ago. Should I look for electrical gremlins (my suspicion), that relay itself, the coil ? Maybe fuel system ? I'm thinking electrical because it doesn't stumble or run rough after start, it just doesn't run.

I jumpered the ballast resistor on the coil temporarily but that didn't do it.
Recently replaced the water pump and before that the elbow off the top of the engine (where thermostat is) but can't imagine how that is related.

Suggestions welcome; maybe I can get on road this weekend - expecting high 70's (unusual but can't complain if driving season is extended)

Look in the area where you found the problem the other time first.

Check for power at the coil + lead (should have two white/lt green wires) with the key on. One wire comes from the starter relay and is powered when the key is in the start position, the other comes from the ballast resistor and should have power when the key is in the on position. If there's no power, then the supply to the ballast resistor or the resistor itself (may have a resistive wire instead of a resistor) is bad. The supply to the ballast is a white/brown coming from the fuse box.

Ignition relay.

There are a couple of relays, check the ignition relay. Also any auto parts store has them and they are not expensive.

You get the prize. It was the same wire that had fallen off its terminal from the ignition relay months ago. But instead of the spade lug coming off the relay contact, this time the spade lug was on and the end of the corroded wire was 1/4" from the connector dangling in mid air.

That was so simple wasn't it. LOL