holley Commander 950 not reading RPM

I have a 950 commander with a MSD ignition, crank trigger, with shielded cable, ecu is seeing 8357 rpm at idle. Occasional proper rpm signal at 5000 rpm for split seconds. Checked all wiring , connections, and grounds, talked to MSD and Holley no one can tell me why. Except a dirty signal. Cranking rpm without engine running seems to be ok.