2003 Chevrolet Cavalier Charging System Problem

After the car is warm it idles at 600rpm, 700-900 when first started, but when warm and at idle ill turn on the a/c and the alternator will not supply enough electricity to keep up with the accessories and it starts running off the battery. After awhile the car will start to stall and eventually turn off when stopping or I'll have to give it gas to keep it going. I'll also notice the a/c runs slower when at a stop. I have already installed a new battery, cleaned the cables, new alternator, idle air control sensor, throttle position sensor, manifold pressure sensor, air filter, and gas filter. Keep in mind when starting when cold it idles at 700+rpms with accesories on and the alternator gauge says it has no problem supplying enough power, only after the car has been running awhile and the idle rpms drop to 600 do I have this problem. How can I fix this problem?