[Keithc8]

Is this a standard shift car? If you have a standard shift and the pressure plate was not balanced on the flywheel and marked this could be your problem. I know most Cobras are standard shift and use a pressure plate. I have seen these pressure plates cause a out of balance problems. If it is an automatic then this will not be an issue. If you are using solid motor mounts you will have some vibrating at different rpms. Good luck, Keith