62 Valiant Speedometer

Where to start? I just got the 62 Wagon running and The other day when I was going down the HWY the at about 60 the speedometer shot up to 100 and then started to bounce around. As I got off the gas to get off the HWY it came back down and stated constant speed which was accorate according to the police speed reading sign that they put out. Then last night as I'm leaveing work when I get up to 20 MPH the speedometer starts bouncing all over again.
Does anyone have any ideas on weither its the gage, cable, or the gears? and How can I check which one first?