my "WAG" is purely based on an acadimic view of the drawings. . .
for the intermidiant lamp,
Loose wire possible (in no particulair order)
the Y wire from starter relay to regulator A
the G/R wire from the alternator to the regulator S
there could be loose wire internal to regulator causing the voltage on the G/R wire to faulter
the transfer relay in the regulator going bad
as for the swinging output voltages. . .
this seems to be common for original as indicated by amp meter. I really wish i was in my prime and had access to investigae this condition as it seems to be the type of problem I enjoyed solving. my guess is it could be midigates with the addition of filtering the stepping field current generated by the mechnaical regulator.
If a solid state, then the regulator is going bad as I would assume field current stabilization is part of that design
given these are just my wild arse guesses, I will bow to any others comments