I just had the same problem with my 2006 F150 FX4 (5.4L), its at the dealer being fixed as we speak. Let me guess, about 60,000 miles on it when it started, correct? What you have encountered is a "Malfunctioning Throttle Body." This is a VERY serious problem, and a pretty expensive problem to fix, its costing me in the range of $550.00 to fix. It's funny though, when I talked to the dealer yesterday they told me they would have it fixed by morning. My question to them was, Why do you have throttle body's in stock for a 2006 F150? These just aren't suppose to just go!!! My truck stalled out on a highway do 65 mph!!! I lost not only my engine but my steering, and my brakes too!! I feel comfortable saying that I have NEVER seen a throttle body fail at 60,000 miles even 150,000 miles. Thats because it is a Life Threatning problem, pretty sad construction on Ford's part and I will be contacting Ford about this issue and I'm going to try to get a refund. We are not the first ones who have had this problem, I talked to another dealer yesterday and thay just had another 2006 F150 5.4L in the shop with the same problem, so this is getting very common. If I can get a hold of someone at Ford I will post the number and contact so that we can get this to be recall. I would think that Ford would want to fix this problem just based on reputation purposes. I will keep you all Posted.