Your Idle Air Control Motor is stuck open. I got a new one at my autoparts store for 71 bucks. My car started idle at 2600 at one time. For a while i just disconnected it and turned up my idle screw.
Get a vacuum gauge they are under 15 bucs.You should have a steady vacuum at idle around 14-18 psi.I think that is the psi its been awhile since I played with vacuum leaks.Mustangs are very known to get leaks along the plastic lines and the vacuum tree on the firewall.Get some throttle body cleaner and spray it anywhere you could have a leak the idle will change. don't touch they will break even if ya look at them wrong.