My speedometer work fine until just before I took the car out of service to paint the underside. It was jumping all over the place. Today I pulled the cable out and looked at it. It appeared to be in good shape, lots of graphite on it. I pulled the cable out of the parts cars in installed it. When I went on my drive the speedometer worked just fine. Any ideas on why I had trouble with it before? I have a spare head (gage) and gear for the transmission