I have an 03 f150 with a 5.4 engine. Truck starts and runs great when cold. Once it is warmed up is when the idle problem shows up. If I stop to run into a store or get gas and then restart the warm engine it won"t idle properly and sometimes will stall 4 or 5 times before staying running. Normally when you start a warm engine the engine comes up to about 1200 RPM for an instant then drops to standard idle around 700RPM. Mine goes back and forth from about 1400 dropping to about 300 RPM. If it doesn't stall, it will go back and forth for about 2 min and slowly level out to an almost steady 700 or so. If I put it in gear during the hunting action, it runs fine and doesn't seem to surge. It idles fine if stopped at a light in gear and takes off OK. I have new plugs and coil packs. No codes show and no lights come on. Any ideas?

With the motor off, Remove the air filter, unhook the air filter housing from the air tunnel. Inside the air filter housing is a maf or map sensor, Use an aerosol can of tech spray for electronics(I use carbutator cleaner, personal preference) and spray the sensor real good, to clean it, and let dry completely. Next, inside the air tunnel is an IAT sensor, spray it as well, and let it dry completely, Next, (if yours is the same as mine) on the back of the intake, Very close to the firewall, is an IAC sensor, (it has a solenoid attached to it), unplug it and remove it withe a 7mm wrench. It will look like a aluminum block with a small plunger through the middle of it to the solenoid , spray it also( outside the vehicle) and let dry. After all is dry, put back together. Before you start your motor! Unhook both battery cables from the battery, Hold them together for30 seconds.without touching the battery. Then put it back together. For the first 10 miles or so, the ECU will relearn. Give this a try, I do it every 40,000 miles on mine. Hope it helps!

The symptom of which you are describing is known as "hunting"
for idle.
You need an idle air bypass solenoid, otherwise known as an IAC. The part mounts right on the throttle body. Sometimes they can be cleaned, but for around fifty dollars, I would suggest a new one.