[Bob Ashlock]

This really sounds like a tachometer-gone-bad situation. The way to confirm this is to (a) make sure the battery voltage at the tach is 12-14VDC, (b) the ground of the tach (relative to the chassis) really is 0V, and finally (c) the tach input signal should be 0 volts when the engine is NOT running. If you have an simple digital voltmeter, confirm that you are measuring 0 Volts DC and then switch the meter to AC and confirm you are still getting 0 Volts. If the meter shows voltage on the AC scale, that would be an indication pulses are arriving that should not be there, and that would mean the problem is coming from elsewhere. (speed control unit, or ?) My suspicion is that the tach is just bad.