If you have the instrument out then you can check if the problem is in the instrument itself.
Take a 3" piece of solid copper wire such as used for house wiring (14, 12 Ga will do), strip and fold one end over to give a double section about 1/2" long. Flatten the folded end so that it will fit diagonally into the square at the back of the speedo. Put the other end in your cordless drill and run it (slowly at first, to make sure you’ve got the direction right). If the needle doesn’t bounce (and the odo and trip count up) then your needle bounce is in the cable, and the worst problem in the speedo head is possibly miscalibration.