1988 Fiero runs rough cold but better as it heats up. Still runs rough compared to before.

Possibly dirty/faulty idle air control valve or dirty throttle plate.

Could also be a faulty throttle position sensor.

To check it you will need a volt multi-meter. It should have three wires going to it. With key in on position one wire should have 5 volts on it at all times. One should be a ground at all times. The other wire should show .5 volts with throttle fully closed and should smoothly climb to 4.5 to 5 volts as you move the throttle to full open position. If it's doing anything other than this it's bad and needs to be replaced.