Have your charging system checked at the auto parts store (free). you might have a bad diode or two in your alt.
BUT..
I had the same issue. The pulley on my Alt was ~3in dia and parts book called for smaller dia (2.5 to 2.75 if memory serves me) smaller Dia = faster Alt speed and more ability to charge at idle. I changed the Alt to one that had the smaller dia pulley, now my ammeter and voltage is rock solid at idle.
I can not say if it was the pulley or the alt but physics says the pulley did the trick.
