I have a 2001 Mustang convertible- 6 cyl. When I start the car, it revs up to 1500 RPM and doesn't come down. It will go down the road at 20 MPH without touching the gas pedal [ gas pedal is not sticking] Have had it to a Ford garage and was told this was normal for a Mustang and nothing could be done about it. Does anyone have an answer to this problem?