There is a relay that is located on the left wheelhouse assembly. I think it is called a TDC sensor. It has a wire that goes from the bottom of the relay to the harmonic balancer. I had the same problem and I replaced the part and the tach no longer jumped. You could try dropping some crazy glue into the metal prongs on the underside of the relay. Sometimes they get loose and you get this problem. In order to remove the relay, turn the black cover counterclockwise and unscrew it off the mounting base. You'll see what I'm referring to. As far as your tranny goes, you could try adjusting the transmission pressure by fine tuning the governor or modulating valves on the side of the transmission, but if you don't have any idea how to do it, then I would recommend you go to someone who does. Good luck.