If you have R12 in your A/C you might want to check the charge on your A/C.
Low Charge = hunting idle
unplug your commpressor and see if it makes a difference.
did with my 88 GT and it went to a steady 650 rpms.
Decided to take the A/C out,got rid of everything except the compressor(I use the compressor as a pulley only,that way I didn't have to change the belt)
I've had a steady idle ever since I took everything out.
I also run with a vacuum guage and that is also running steady at idle.
Gained on horsepower and fuel mileage.