that sounds like an electricky problem. Perhaps a wire is not connected properly?? if any of the 3 are loose or dodgy, then you would get the intermittant signal making the needle jump around, but about it hitting 8000, well unless you car does these revs, then i would point the finger at the pcb on the back of the taco. Take it to a instrument shop and have them test it if you are sure the wires are all in good nick.