Stepper motor in the instrument cluster sounds like it has gone faulty. Look for a speedometer repair service in your area. I had all the stepper motors in my cluster replaced for $100 in Tulsa OK when I 1st got my 05 GT the speedometer was INOP.
This is not an uncommon problem with the 05 model year. The stepper motor is faulty and Ford replaced it in 06. If you're into modding your car you can buy the stepper motor on ebay, remove and disassemble your instrument cluster and replace the old motor. Not a terribly difficult job; you'll just have to be sure to reset and test all your gauge needles back to the right position.