I've had this problem for 3 years now and I still can't figure it out. Battery is a new Optima Yellow (Did it with the old Interstate as well) We have tried 3 alternators. (We even tried the one out of my Formy which we know is good)

We start up the 89 Camaro and the Volts are perfect for about 10 minutes. After using the lights or brakes the volts DROP to about 8-12. Turn off everything and drive and it may move up to 13 Volts. We tested all 3 alts in the car and they all read bad. Said they were putting out only like 12 amps max 40 some. Put all 3 alts in the Formy and all were good. we have brand new cables from the back of the alt to the battery, battery to starter, battery to a good ground. I am cluless. Any suggs?