Good point, Larry! Did he disconnect the wiring at the tach head, or at the distributor? There's a definite possibility that the tach trigger lead might have an intermittent short or a high-resistance connection to ground somewhere between the engine and the gauge console.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!