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.
2006 GT Redfire 5 Spd Manual. Saleen S/C. Custom Diablo tune by Adam @ ST Motorsports, San Bernardino, CA.