I agree that the Idle Air Control Valve is the most likely cause. You may need to remove it from the car and carefully clean it out with some carb cleaner. Work the gate back and forth gently using a screw driver.

Note: With the the 1989-93 IAC valves, you must attempt to close the small slit visible, and then a larger opening will appear on the far side of the gate. It is spring loaded. The 1985-1988 IAC valves are not spring loaded and will freely twist open and closed with a flick of the wrist, and they are not interchangeable.

Once you've cleaned it, lubricate it with some lightweight oil. If you skip the lubrication step, the problem will probably not be resolved. I'm suspect your idle oscillations are because the IAC valve can not keep pace with the command it's getting from the ECU, -probably due to grime and gunk.

During this entire process, try to hold the valve assembly vertical, with the electrical connector pointed straight upwards at the sky. This should prevent carb cleaner, etc, from getting past the shaft seal and down into the motor assembly. -I've burned one of these out that way.

God bless and have a great day,
Fitz Fitzgerald.