Thanks for the comments. It could have been one multi-wire circuit. I didn't trace out all of the connections and its been a few years - about 4 years since I respliced the last neutral.
However, the problems happened repeatedly in 4 different houses - three homes were owned by me and one was a neighbor's.
In most cases, when the neutral opened, one or more outlets simply went dead.
In one case however, when a neutral opened, the return current tried flowing thru the BX ground, got to a light switch box where the clamp screws may have been loose, made the switch cover plate screws hot and scorched the wall. Luckily there was no greater fire. That's went I went into high speed mode replacing the soldered neutral pigtails in as many houses as possible.
I went room by room in each house, checked every visible switchbox, outlet box or junction box. ALL of the junctions were made in the boxes for the overhead lghts.
In the time since, there has never seemed to be a neutral overload issue.
Doug