1990 Nissan Sentra XE shuts off

[Snakepilot]

My 1990 Sentra XE has performed perfectly until about 10 days ago. Since then it has shut off, suddenly, 3 times.
The car is in excellent condition ( everything works ) and is well maintained. It has 102K in mileage.
The first time it happened,I started the car after being it was in the garage all night and in backing out of the driveway it shut-off. No apparant reason. Then 3 days after the first incident, after starting the car in the morning and driving a mile then parking an hour then driving approx. 1/4 mile, it shut down again , ditto. Then last night after being parked all day it started then shut off. In all cases after waiting approx. 5 min it started and ran fine.
What is going on ? Is the problem in the electronic control unit ? The distributor ? Where ?