my 88GT runs great, but when i turn on the air or heat it wants to cut out at first, i have even upgraded to a higher amp alternator an it still does this.

 

You mean the engine wants to stall? If so, try cleaning your IAC motor and your throttle body. That might do it.

 

Turning on your heater (at first) draws a huge amount of current from the electrical system. Do an experiment - sit in your driveway with your car idling, and turn on EVERYTHING (but the heat) that will draw from the system - radio, headlights, dome lamp, foot on brake, rolling both windows up and down, cig lighter, etc) and watch your voltmeter. Does your car want to studder? Then turn all of that off, and simply turn on your heater.. and watch. Your RPMs will probably drop and your voltmeter will go way down. Depending on where your idle is set, it may or may not come back up.

Now, go out and drive on the highway, cruise at 2500 RPM. When you're steady at 2500ish, turn on that heater. Chances are you'll see your voltmeter dip way down, but then imediately go back up. That's fine - that just means your alternator is doing it's job.

FWIW, on my Mustang, I have to be careful with this as well. I've found that if i'm running EVERYTHING at the same time (including the heater, and rolling the windows up and down etc) with my car idling in D at about 1000 RPM, it will drain the charge faster then the alternator can keep everything going. However, the situations are very rare when you'll find yourself sitting in your car with your radio full blast, foot on brake in D with both dome lights and the cigl ighter on, headlights, heater full blast, and rolling your windows up and down.