Update:

So far I've had about a half-dozen suggestions (some on-list, some
off-list).

3 of you have suggested the Hall Effect Sensor.      It so happens
that when I checked the cap & rotor, I did notice that the Hall
Effect sensor connector seemed loose (didn't seem to fit very tightly
in the distributor), and the electrical plug to the HE sensor didn't
seem very tight either.

Not a cheap part to replace on a whim, tho...   Any opinions on
replacing the HE sensor alone, vs. replacing the entire distributor?

I'm newly moved to the bay area (Oakland) -- can anyone recommend a
local part shop that would have 'em in stock?  Dealer wants 5 days
and $186 for the HE Sensor alone.    Vanagain and Busdepot have
entire distributors (rebuilt) for around $150...