Don't go throwing parts at it. Check it out. I bet you have a vaccuum leak. Very common failure area on this truck is the pipe from the pcv valve to the back of the throttle body. There is a 90 degree rubber boot on the back of the throttle body that commonly fails. Follow the pipe from the pcv valve back there with your hand. Bet you'll find it is dry rotted & cracked, especially if you've never replaced it. New assembly is about $30 from Ford-comes with new pcv valve, pipe, & rubber.