I had an erratic tach signal. My car would start but the idle was all over the place. I fixed it by using a shielded wire for the tach signal. I used shielded speaker wire. You ground the shield at one end (leave the other disconnected) and I coiled the original wire up and his it in the plastic casing I was using. I didn't want to cut it in case I had to return anything. Also route the wire away from plug wires and distributor. I am not saying this will fix it but it's worth a try and it fixed my idle problem.