Help with 05 gto alternator!
 

Ive got an 05 gto and like 2 months ago my "check alternator" light would come on then turn back off after about 10 seconds about once a week but i never really paid any attention to it until one day the light never came off. I took it to a local auto electric shop to get it looked at and they told me that it looks like the regulator was going out. They went ahead and rebuilt the whole alternator besides the pulley mechanism that goes inside because they could'nt get their hands on one, at least that is what they said. They also checked the battery and everything tested ok. Well the "check alternator" still comes on, I took the alternator to a different electric shop and they also told me that the alternator is testing good. They test the wire coming from the computer and said that the computer was fine since it was sending 5v from one of the wires. What is strange is that the light only comes on when im driving in small streets or im stopped, once I jump on the highway the light comes off. I hope someone can tell me what might be wrong with my goat.. by the way I also bought a new battery to be on the safe side.

Under drive Pulley?