Speedometer with a mind if it's own

I'm back with the 91 Grand Prix, You can have the car running and in park and when you press on the break pedel the speedometer will have you going anywhere from 20 to 40 mph. To add to the confusion. The oddometer will all of a sudden out of the blue take off like a bat out of hell and add a few hundred miles to the oddometer. Christine has nothing over on this car...