2002 Pontiac Firebird voltage drop

Hello...
When I have my a/c on high and when I hit he brakes the voltage drops to 12 1/2 and the a/c shuts down, you have to turn it off and then back on for it to work again. the dealership said that the alt. and battery were fine and they could not find anything wrong...when that happens it takes it awhile for the volts to pick back up to 14 volts. thanks for the help...