"Pulsing" voltage, RPM drop with load.

Semi normal at idle. My bet from personal experience is the alt is not being driven fast enough at idle. Ether the alt pulley is to large dia or crank pulley is too small dia or both. My bet is on the alt pulley unless the engine has been Frankensteined at some point in its life, as the oe alt has most likely been changed a time or two.

What measurements do you get for pulley dia?
Also have you had the charging system checked at the auto parts store, might show a bad diode, or the like.