I recently purchased a 67 XR7 GT w/ 390 auto, and I'm tinkering with all the little stuff as I go. Next up on the list: The speedometer bounces wildly from 0 to 70 at speeds under about 45 mph. get above 45 and it's smooth and accurate. Odometer is spot on w/ mile markers on e-way, so I'm assuming the cable is good. Any ideas before I tear into this one?