IAC(idle air control) Valve is how computer sets warm and cold idle RPMs.
It should be cleaned every 5 years or so.
Just 2 bolts.
read here for details: How to: IAC valve cleaning thread (w/pics) - Ford Explorer and Ranger Forums "Serious Explorations"®
4.0l OHV engines eat most brands of spark plugs, Motorcraft seem to last the longest, then Autolite.
Never use single platinum spark plugs on distributorless Fords, regular or double platinum only, single is a waste of money.
Worn out spark plugs cause rough idles.
My '94 spark plug wires lasted 18 years, but they did eventually start to cause misfires, poor idle.