Often times, an idle related issue is caused by a vacuum leak. That is the first thing I would recommend checking. Here is a link that explains how it is done:
Also, your vehicle has what is called an idle air control valve. It is located on the throttle body and is responsible for maintaining a steady idle speed. If you find there are no vacuum leaks, that is the next thing I would check. However, before I can provide additional information related to it, I need to know if the engine is a 5.0L or the 5.8L.
I attached a pic below and highlighted the IAC and the EGR. Check around the EGR for any vacuum issues as well. Next, take a look at the IAC and make sure the electrical connector is tight, not corroded, and not damaged.
One last thing. Your vehicle has an OBD1 computer system. It would be a good idea to see if there are diagnostic trouble codes stored. Here is a link that explains how to retrieve them from this system. You will not need a scan tool. However, you will likely need a test light:
Let me know if any of this helps or if you have other questions.
Friday, December 11th, 2020 AT 8:25 PM