Fuel system

There is no way we can tell if the engine computer is requesting the higher idle speed in response to something, or idle speed is too high and the computer is trying to bring it down, but with limited success. You need a scanner so you can look at the sensor data the computer is seeing and responding to. In particular, the coolant temperature sensor is a good suspect. Normally they have a very low failure rate because there is just one component inside them, but Ford had a huge failure rate in the early 1990's. The symptom was erratic idle speed. You would see this on the scanner as the coolant temperature reading bouncing around, and the signal voltage is jumping around and is not steady.

Also, on any car other than a Chrysler product, look at the fresh air tube between the mass air flow sensor and throttle body. There cannot be any loose hose clamps, cracks, or other air leaks or vacuum leaks. If any air sneaks in that does not go through the sensor, it will not get included in the fuel metering calculations. A vacuum leak will result in increased idle speed that the computer may not be able to adjust for.