5kS cutting out, tachometer drops sharply...

[rochow@pixelmagic.com]

After an uneventful 30-mile interstate drive and a few minutes
in town traffic, my '84 5kS suddenly started to cut out for
fractions of a second at a time; at low speeds/low rpms,
the tacho needle would actually jump down to 0 every
time (at higher speeds, it wouldn't get that far, and the
speed of the car made the engine's dropping less obvious,
while the tacho still twitched rather a bit).
>From the tachometer, I'm guessing lack of spark electricity,
occuring at a point before the tachometer gets its signal.

So, where does the tachometer get its signal?

tia
Benedikt Rochow