Ford Repair: 2001 F150 - erratic idle when heat/cool is on, couple questions, lurch

Question
QUESTION: I have a 2001 F150 with the 5.4L V8 in it. I have recently noticed that the idle is very erratic when I am using the A/C or heat. Sometimes when stopped it will surge to the point of causing the vehicle to lurch forward if you foot is not firmly on the brake. Other times it drops to the point of almost stalling. This only happens when the running the A/C or heat. If I shut it off, the idle is very consistent. Any suggestions are appreciated.

ANSWER: couple questions, how many miles are on the truck, is the check engine light on or have you had ant a/c work done to the vehicle in the last month or so ? thank you.. -b-

---------- FOLLOW-UP ----------

QUESTION: The truck has 71,000 miles on it, no check engine or any other lights on, and there has been no work done on the a/c.

Answer
o.k. i need you to do one thing, when you have the heat on, turn it to the floor only. this should stop the a/c compressor from running. my guess what's happening is, your a/c compressor is about to go bad. it's struggling to work. your computer is trying to compensate for the load the a/c compressor is putting on the engine by cranking up the idle. when the compressor shuts off, the computer has cranked the idle so high the car wants to lurch forward. but try the above first and get back to me.. thank you  -b-