Ford OBD 1 system, these wee bad for failed ACT sensors and IAC motors, this one sounds like the IAC motor has quit, now you can try removing it and cleaning it up and the passages and putting it back on. No remember these old systems had to relearn after the power was disconnected, so I would disconnect the negative battery terminal then turn on the headlight switch for a few minutes, then turn off the headlight switch, reconnect the battery terminal, set the park brake and block the wheels start the engine and put it in drive and turn on every system, headlights, heater fan, etc. and see if it will stabilize the idle after about 5 minutes, if it does then drive normally for at least 10 minutes. If you can't get the idle to set right then you may have some sensors misreading. You need to find a shop that has a Ford breakout box and the older equipment to diagnose this system. Sitting is a very bad thing on any computer controlled vehicle.