I have a '92 with similar symptoms. At the time, the plugs were new, clean air filter, and the A/C was also off. The dealer immediately told me it was the Electronic Air Control and wanted to replace it. Before agreeing to replace the unit, I read through the shop manual.
The manual suggested increasing the idle before replacing the unit. I increased the idle by approx two turns (if I remember correctly, I had to turn the idle counter clockwise) to compensate for the rough idle. My car idles perfectly at the correct RPM now. Problem solved.