To set your idle screw, back it off until you can fit a .010" feeler gauge between it and the stop. Tighten the screw until you can remove the feeler with a little friction. Then turn the screw in for another 1 1/2 turns. Disconnect the battery for 15 minutes and reconnect it. Start the engine and let it idle with all accessories off until the idle smooths out. When it does, let it idle for an additional 2 minutes. Turn the car off and then start it again with all accessories on. Do the same as you did with them all off. Turn off the accessories you don't need and take it for a spin.
If the idle doesn't smooth out after 10 minutes, it probably is the IAB valve. Try cleaning it.
Let us know what happens.
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351W 89 Mustang GT Convertible