speedometer stopped working?

2004 f150 speedometer now rests at the 120 mph. When I come to a stop it moves to abouyt 70 mph. Then as i drive it moves to 120 and dances.

Some 2004 ford trucks have a problem with the wiring harness used for cruise control runs too close to the alternator causing EMI. At a stand still it will look as though you are going 30 mph. If your alt is putting out an exessive amount of current, this will cause the dash module to become overwhelmed with the EMI and eventually fail.