Your problem is very simple.
The R or Relay terminal on the alternator produces an output only when the alternator field is energized. The field, in turn, is energized and de-energized by the regulator, based on need. If the batteries need charging or there is a load of any kind, such as the clearance lights, then the field will essentially be energized all the time, and your tach will work fine.
If, however, the batteries are fully charged and there is no other load on the system, the regulator will be rapidly switching the field on and off. It will probably not be "off" long enough at any time for the "Not Gen" or "Charge" light to come on, but it will wreak havoc on the tach reading.
The cure is simple: Drive with your lights on. I recommend this practice for safety anyway.