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.