if you had a bad valve spring ( broke) i DO NOT think you would just see it in the tach ..but also hear and feel it in the way your motor runs,sounds etc...
i will say the tach has a bad ground,or some sort of electrical problems with the tach..even the tach could be bad....they do not live forever...before i would go tearing stuff apart or assuming the worse..get ( borrow ) another tach and plug it up..if you got the same problem then go from there...try to keep it as simple as possible from the start....from all my years experience most major problems turn out to be minor ones...
