my opinions:

-Stock alts still being new back on the days where unneficent at iddle. In most cases you disconect the batt with engine running and engine will stall. Alt don't provide enough juice at iddle.
-If you add to this a low tension on alt belt... increases the problem
-then, add to this years of sulfating, oxidation, loosen contacts, damages on wiring (and ammeter itself) between batt and the main power splice which is located between ammeter and alt.

we have the perfect storm to an ignition system failure ( along with the rest of the car electrocity ) with everything of this working together.

sure if you have an alt able to source the complete car electrical system at iddle, you barelly will notice any problem if some, between batt and main splice, because alt will be feeding the main splice without any reading on ammeter... straight to all the devices