I have the same aftermarket Tach, mine bounces as do many other owners of the same tach. They are renowned for it. Those tachs seem to be sensitive to dirty/noisy voltage. I did have some degree of success with adding a resistor to the tach wire inline from the ignition to the tach. but partly the dirty voltage signal could be coming from a connection and your tach is just mimicking what you are seeing in your headlight..