Alternator Intermittently Charges the Battery

[Christopher Gharibo]

Another issue on the '91 CQ: the alternator will stop recharging the battery intermittently. The voltage will drop from 13 to 10V and the battery light will come on and the car will run on battery power. 

The alternator brushes are almost-new and the belt is tight. The alternator is about 15 years old. 

What's the next step? New alternator?

Thank you. 

CG