Sounds like your CIS (Constant Idle System) is not working. This is a part of the 740 computer that regulates a small motor under the intake manifold than adds idle air when needed. Your motor may be shot, you could have a bad electrical connection or perhaps a hole in the hose between the motor and the intake manifold. There is also a serrated thumb wheel under the throttle control body for setting the idle speed. The way to set the CIS is to ground the red and white wire connector on the right fender, set the desired idle rpm with the thumb wheel and then remove the ground connector. This will activate the CIS and it will keep the idle 25-50 rpm above what you set. The CIS will also increase the idle when the AC compressor cuts in.