What Scotty said, and if I'm not mistaken your F150 has a configuration like the diagram below where the MAF sensor is way up by the intake box and then you have that long hose to the throttle body which, at the age of your vehicle, may have some cracks in it where "unreported air" is entering the intake downstream from the MAF sensor.

So do the general vacuum leak test, but I'd pull that hose, inspect it for dry rotting, cracking, and airtight connections at both ends.

While you have it off clean your throttle body (throttle body cleaner) and your MAF sensor (MAF cleaner)

Try my video make your car run better with a little spray cleaner and also check for vacuum leaks