I had this issue on an old car of mine. At a certain point the tach would jump around. It ended up being a bad control module. One way to check. Does it do it if you are at a stand still? If yes have someone rev the engine until it happens. Place your hand at the tail pipe and feel the exhaust. It should be a constant feel from the cylinders. If you have a miss you should be able to feel it. If you have any questions let me know.