The oven thing may be what we call and MSL problem. Shouldn't need it.
BTW, are you talking about a Smith and Wesson or Stewart Warner?
May I suggest a shielded cable from the dist/box to the tach. You may be picking up noise that tach is reading as the signal. They can also mix and lower the tach reading. If it is consistant, the tach will consistantly ready wrong.
It just a suggestion.
