I have a holley sniper shift light that just recently started jumping all over with rpm reading. I have put 2 other tachs on it with little rpm variance no drop to zero. I have installed a new distributor, TFI, coil. I also have monitored voltage at the coil on the + side and same voltage as battery when running no indication of dropping. I have rewired the shift light with all new wiring and still does it. Weird thing is when I switch over to my Summit ignition box, with either coil, the shift lite no longer does it all. MAYBE a bad poorly adjusted ignition switch? I'm at a loss. please help.