You may not have installed it fully engaged at the speedo. Remove and do the install again.
Also, could you have rerouted it, introducing a new bend that wasn't previously there?
Bouncing needle usually is a result of a binding of the inner section on an internally worn and rough outer jacket, or a bend that is too tight.
But as it is a new issue, I'd first check everything you did to the cable in your recent endeavors.
Larry
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