My 88 bronco will surge slightly at idle,it has a high idle when cold and wont surge but will surge for a few seconds when the motor is warm. It was much worse before I changed the pump in the tank,I do need to change the thermostat since I think the PO put in a 160 stat but will probably have to change the intake due to the bolts will break. If getting a correct thermostat does not help the slight surge what else should I look at and it does not turn on the CEL,EGR is stuck tight and solenoid not working so I doubt that is the problem but I will replace both soon.

 

Try cleaning the IAC valve just inside the throttle body. It's bolted to the side. It gets dirty and sticks.