Something like that happened with my speedometer when I installed a new cable(20 years ago). I loosened the cable on the speedometer and the problem went away. I didn't like that the cable was not screwed tightly to the speedometer, but it fixed the problem. Recently, I rebuilt my speedometer and found a couple issues that you could check. If you want to open up the speedometer, make sure the speedcup is not binding. You can rotate the speedcup and the spring should return it to rest position. The speedcup assembly has 4 screws, if there is a bind, you can loosen the screws and might be able to shift the assembly so the speedcup moves freely, and then tighten the screws. The steel spindle is soldered to the aluminum speedcup, that can break. Make sure the spindle is not independent of the speedcup, it can be bad but have enough friction to appear good.
