This happens all the time. Just take the iac off the throttle body and clean the inside with carb cleaner get all inside the ports and around the piston. Then reinstall and drive.
Is problem fixed? Drive it for a few months and see if ti starts acting up if so clean the intake and check the valve seals and clean the iac again.
If it doesnt fix it replace the iac.
Simple its always the same thing dont hurt your self looking for any other cause. This happens to all 5.0 80s fords. Single or dual throttle body. Car and truck.
Hope this helps. I see your last photo posted is what looks like the iac so i am assuming you got a new one.