The IAC (idle air control solenoid) might be dirty and sticking.

Its the solenoid that is under the upper intake....near the fuel pressure regulator. There are two air hoses that attach to it, one that goes up to the intake air tubing on the throttle body and the other goes to the intake manifold. It takes fresh clean air from the TB tubing and channels it around the throttle plate and into the intake manifold. The more it opens then the higher the idle.
It is controlled by the computer. The comp. looks at the temp of the engine and the RPM (probably some other things too)to regulate it.

Sometimes they get sticky and cause the problem you are having.

There are some posts in the archives that talk about how to remove and clean it.

I use a foamy throttle body cleaner that I get from the Ford dealer.
I just remove the hose where it attaches to the TB tubing and while revving the engine I spray into the hose. If the solenoid is working, the cleaner will be drawn into it and then into the engine intake.

I do this as a maintenance procedure.

Good luck.

Kyle245