I have had this issue before on a couple of my stangs. What usually fixes it is a new TPS (throttle positioning sensor). But just word for the wise, I have bought new TPS's and it still did the high idle, but I would buy them from places like AutoZone, but then I went to Ford and replaced it with a OEM and it worked fine. But keep in mind that just cause a part is new does not mean it is a good part. Sometimes you have to return the new part for another. But the price from Ford and the auto parts store was practically the same. But by first hand experience with this problem a few times, a TPS is in your future. And they are cheap to replace and takes all of 5 minutes to do the job. I by nature always disconnect the battery whenever I am replacing an electrical part. Would suggest the same to you. Hope this helps.