I have a 91 GT with 140K miles and I've been experiencing a voltage issue since I purchased the car in 2005.  While sitting at idle with no lights or fan running, or while driving the voltage stays in the normal range of the guage, however if you stop and you have the fan or headlights turned on, the voltage guage drops.  Then if you turn on a turn signal, the voltage drops further with every blink of the light.
I have recently replace the alternator and battery, but not sure where else to go.  In fact I just replaced the altenator and the guage fluctuation became worse.  The altenator is however charging the battery sufficiently.  Has anyone else seen this issue, and if so how did you fix it?  

You seem to have a very sensitive gauge. However, if you can determine what the system voltage is with all of the loads applied, (multitmeter hooked accross the battery terminals) and it's above 12 volts, why worry about it?  

I did just that this weekend and sure enough the voltage stays above 12.  At just an idle, the voltage is almost 14 volts and by the time you add the heater/air conditioning fan, full lights, and flashers, the voltage is at about 12 1/2 volts.  My concern was that everything in the car seems to dim or slow down with all of these items working so I figured that there is a problem.