My tach is always reading higher than it should, but it bounces around. Also, my radio cuts out intermittently. Bumps will affect the radio as will shifting sometimes. I'm thinking I have a grounding problem and need to replace grounding wires. Can anyone confirm?

 

Tach not reading accurately is likely bad capacitors in the tachometer.

Tach bouncing around, especially if it happens the same time the radio is going in and out, is likely due to a ground issue in the dash harness.

I've also had issues with a tach bouncing around where it was the igniter in the distributor going bad. This is where the tach signal originates from.