Do you have mechanical points type, or electronic voltage regulator?
Some mechanical regulators can be adjusted to provide more or less charge. It is a bit of a PIA, and temperature plays a part in the adjustment…
Electronic regulators are preset, so it comes down to if it works or not. That said, other contributing factors enter into how well a good regulator will regulate based on the voltage it is reading in the electrical system. If it is seeing high voltage, doubtful, but possible, it will throttle down the alternator’s current out-put.
If regulator sees low voltage, a more common problem caused by poor connections, switches, and bad wiring, the regulator will step up the current flow based on faulty information.
Do headlights go bright than dim when car is idling in gear, and other electrical loads are present such as heater motor on high, wipers on, air conditioning running, electric radiator cooling fan, or big high power sound system cranked up?
I think that your alternator may have trouble keeping up at low rpm. As suggested previously, bump up idle speed 50 to 100 rpm and see if the problem goes away. This may or may not be a proper fix.
One thing you can do is remove the alternator, take it some place for testing, which should determine any deficiencies.
On the other side of the coin, the harness feeding the motor may have developed a voltage drop, and or motor has dragging bearings. Or, you have too many high draw electrical devices that are not stock loads competing with the heater fan.
