Well the gremlins started Showing this week. Tack gauge drops to zero than back to normal. I suspected it was the Alternator, but plugged in an obdii port and my diagnostic shows no RPM drop when the analoge gauge fails. I assume it's only in the gauge. Also autozone checked the battery and alternator, said it looked ok .Anybody ever had this issue and it not be the alternator?

 

The small wire that plugs in (not bolts down) to the back of the alternator takes the signal to the tach. It could be open or grounded and cause your problem. There is also a circuit board inside the cluster that controls the tach from that signal.

 

Looks like the wire terminal on the alternator was not secure. 8mm bolt works great now.