I'm running a parts store reman 3G (Autolite brand from Shucks) listed for a '95 GT. Swapped the huge pulley it came with for my 2G pulley and added an additional 4G ground wire. I'm not running underdrives and I have yet to install my Taurus fan, but at idle with all acessories on the volt gauge doesn't budge, I have no dimming of any lights, and my windows roll up and down faster than they ever did with my old 2G alt. I've been driving the car all winter and even with the heater, rear defrost, headlights, and wipers running all at once every day it's been working fine. I also have an aftermarket stereo that puts out about 500 watts, and even with the music turned up it doesn't seem to tax the alt.
I'd say either you got a bad alt, or maybe you have a weak ground someplace. I bought 10ft of 4 gauge cable and replaced all of my stock grounds as one of the cables was coming loose in it's ring terminal and they were all looking pretty ratty anyway. I bought solid copper ring terminals from Lowes and soldered them to the cable ends. They look good and should last for years.