battery charge problem

On my 1987 Fiero SE I have had problems charging my battery to its full potential. It starts at a normal voltage 12-13 volts. But, after the car has warmed up and ran, the voltage drops down to about 9 volts and remains there. I have replaced the battery and alternator with new ones but it still does the same exact thing. I also had the new battery and altenator load tested just to make sure they were good. They tested good. I'm just hoping that someone may be able to steer me in right direction.

i have had battery cables rotted on the inside so you could not see the damage and have symtoms like yours. also, ground connections could be suspect.
good luck