It might be a bad diode trio - more accurately, one bad diode of the three. That often allows enough to keep the car going but you don't have enough voltage to keep up under certain loads. I had one go bad and I took my alternator to an alternator shop and they rebuilt it - using a good used diode trio from the 94 NA I stripped. If you can get a shop to rebuild yours, at least you will be sure to have the right pulley (since you already have it). Buying a replacement can often get you the wrong pulley and cause additional pain trying to stop the squealing belt.