What I'd suggest is disconnect all wires from the tack & connect it back up w several jumper wires & see what you get. This'll take a possible intermittent open in the wiring out of the equation & only leave the tach tho u sure wouldn't think 2 tachs in a row would act up in short order but nothing on the rest of the vehicle electrical/electronic system has been changed that could possible have affected this so might as well k.i.s.s. & see if we can narrow it down EDIT I'd go red feed directly to starter relay, black neg to batt neg, (prob green) sense wire directly to coil pri neg