Whats wrong with my alt.

[93HATCH240]

Hey guys I have a 95 motor with a 93 alt. The alt was replaced by the previous owner about a year ago. Ive had the car for about 9 months. The battery is also about a month old. The problem is after I let the car idle for a while about 15 mins. All the lights dim out and my radio cuts in and out and shuts off by itself. The second i start driving it everything is fine and dandy. Its never had a problem starting up in the morning so im pretty sure the alt is good and still able to charge the battery. I checked all the connections and they all look good. Also the belts are still good. Any ideas?I tried to search but didnt come up with much.
Modified by 93HATCH240 at 8:49 AM 5/11/2007

[TougeMS13]

I would pull the Alt and have it tested at kragen or something for free. Some aftermarket alts are cheesy as hell.