What six-shooter said. The ECM REF+ input is a 5V digital signal. Be sure to run a ground to the REF- input of the ECM (pin B3). This ground should originate at the same location as the Hall Effect sensor ground.

The reason for the varying RPM readings is that the ECM calculates the RPM via the time between distributor reference pulses (DRPs). With the odd fire setup this varies between the pole pieces.

RBob.