im suspecting the alternator diodes is somewhat involved...
the flickering headlights and the dropping voltage, plus the increasing instances that the headlights will flicker...

the alternator may not be sensing the load, and therfore does not provide the needed charge...

as to what the casa is insisting," that the unit is ok because there is no error yet happening..."
its because the vehicle is still within the range of charging (12.7 volts) but lets put that into perspective;

if a water pump is designed to pump at 100 liters/ hour, if it only performs at 70 liter/hour? does that constitutes as "walang sira o diperensiya"

just point out sir to the SA, na with other montero even with load, charge rate is at 13.7 volts....
that, being in the range of 12.7 volts of charging is not acceptable.!!!