Vertex Magnetos

[chevythunder]

I recently purchased a car with a Vertex Magneto set up in it. My quesiton is how do I stop the tach from jumping around (a lot) during driving? At idle it seems fine but will jump around 2,000 rpms during speeds, especially higher speeds. I checked the paper work that came with the car and and it indicates that a signal converter is installed. I also believe the tach is grounded to the frame but not positive about that. Any thoughts? Would it be easier to just change over to a conventional distributor and coil?