Well I am quite certain that you are relying on garb gauges. First thing I would do is check your starter solenoid terminals. One night I was driving home and my car completely died, the power to even my gauges was dimming. Turned out that the nut that goes around the terminals was extremely loose and didn't allow enough contact to the alternator wire so I was just using power and not recharging.

 

go to a car audio store ask them for a (2 Farad power cap) it will take some strain off your alt and make your subs hit harder there pretty cheep

check ths out .. http://cgi.ebay.com/ebaymotors/LEGAC...QQcmdZViewItem

 

have your alternator tested, sounds like you got a defective one (it happens) or you have the wrong alternator, the more stuff you turn on the more amps your alt. is supposed to put out, sounds like it cant keep up.

 

i have a 95 gt and the same thing was happening to me i changed the bat and alt and it would still go low. i ended up changing the belt that helped for a while. but come to find out my smog pump wet bad and it was slowing everything down. so i just took it off. and rerouted to belt. i also went ahead and put underdrive pullies on. this past weekend i drove the car 5 hundred miles and didn't have a problem. my be something to think about. how many miles are on the car? hope i helped but it could have been cause my car has 150000 on it.

 

update: went to autozone, had them give me a new alternator since i got the warranty for the other one. However, since im at school, i dont have any tools so i had to get it installed at a shop [:@] im crossing my fingers that this is the problem