I have a 91 GT, and if my air conditioning is on and i'm slowing down to stop it drops my voltage low and my rpm drops low too. It just about stalls my car. Lights play a factor too. If there on at the same time its worse. I don't think my alternator is the problem because my battery is never dead and it runs great if i'm not using those two things, and i need the ac because i live in Texas. It gets HOT in Texas!!! I'm not sure what it is?