Flaky tack

Just replaced the Summit racing tach in my 64 Polara that's been working fine for 5 years. Figured I got my money's worth. Needle started fluctuating at idle, then jumping to 7 grand when I blipped the gas a little. I bought a new Sunpro 5" tach, which started doing the same thing immediately. I replaced the coil, ground, and power supply wires, but a short blast on the gas will rev the tach to 9 grand. Am I missing something obvious here? It would appear the tack is not the issue since both tachs displayed the same symptoms. 426 wedge motor, elec. ign., Accel coil. Is something in my electrical system going bad? The car runs fine.

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

something else simple, my sunpro tach had a switch to change from a 4cyl(or 6cyl?) to a v8 tach. If yours has the same make sure its set to a v8

roe