Weird voltage drop with AC on. Advice?

I had the same problem with my coupe. Turned out to be a bad connection between the 10 ga charging wire and the ring terminal on the alternator. I had just crimped it and it couldnt handle the current draw with the AC on. Learned my lesson. All terminals are soldered now.

 

This is my opinion...take it for whats its worth. You can spend countless hours and dollars trying to fix your vehicle yourself.... or you could take it to a professional and have them diagnose the vehicle and be done with it. Its hard as hell sometimes to diag a car "over the net". I promise you that if you were here in Austin and had the vehicle in front of me I could tell ya what is wrong with it and what it would take to fix it.

 

Well, I found it! Belt was slipping just enough to cause trouble without appearing loose or making noise. Also, the factory diagram for the regulator plug doesn't specify "component end view" or "connector end view", so that was backwards. The regulator pinout is symetrical. I_I shaped, in a circular housing.

Thanks for all the help out there! Mystery solved!