Speedo is mechanical, its possible the cable is damaged or there is a problem with the speedo itself. Mine does it even with a new cable fitted.

Rev counter and missfire, possibly the symptom of the same problem or two individual issues. I would be addressing the missfire first, check the condition of the basics. Coil, plugs, plug gap, leads, cap and rotor arm. Then if that doesnt solve the issue, or both, I would be inclined to spend some time with a good quality multi meter and check the engines sensors at the ecu plug.