Today I drove the car about 25 miles. Occasional minor hiccups and one stumble that was almost a stall, all at low engine speed when the throttle was opened to accelerate. Otherwise ran normally and the tach was steady.

I don't think the problem is with the electrical system as a whole (voltage regulator or whatever) as the headlights, dash lights and all gauges other than the tach (including voltmeter) have remained normal the whole time.

What I am trying to figure out is whether it is the power stage, the Hall effect sensor or something else entirely (resistor in the rotor perhaps?) which is very difficult to determine when it is not having the problem. I have a spare power stage, and it's easy enough to change on the side of the road. I also have a spare distributor, which is not.

I might go ahead and do a tuneup---it's been about a year and 15K miles on the NGKs