1990 Ford Mustang Idles to high

My mustang has had an idling problem on and off for the last several thousand miles. The car is a 5.0, five speed with air and 78000 miles. The engine will jump to 2,000 RPMs and hold there at stops and between shifts and will last until I stop. However, almost every time I turn the ignition switch off and kill all power it will idle back down to around 700 RPMs. There seems to be no pattern, but more often when the engine is cold. It has done this with a warm engine and in the summer. Several mechanics have looked at with no resolution or probable cause.

Thank you

Carl Hodges
chodges@nc. Rr. Com