I believe this is a known problem for mustang, my gt had the problem. But the cobra is worst. I turn the ac on to vent no voltage drop, then to actually ac or max and the voltage and idle drops big time. Some time the car just shuts off. What would cause this???
that is the A/C actually turning on. When you put it on vent you do not run the A/C compressor at all... You are jsut pulling air in from outside and pushing it out the dash...
I would look into making sure the A/C is properly charged...
My guess would be your IAC needs a good cleaning. It's probably not drawing too much power so much as a dirty IAC isn't allowing the computer to compensate for the additional load, so the engine dies when the idle speed drops.