Hope this is helpful; Check the spacing on the "mag pickup" that "counts "beans" on your front pulley.. Course, thats after you check all the appropriate wiring connections. Some of them, especially @ the drive pulley, are in a rather harsh enviornment!
I spent a zillion years as a military maintenance test pilot, mostly on helos ( AH-1 G thru FMC, and AH 64 A, and a couple D Apaches). That's virtually the same Mag Pickup we used on Chadwick and Rads Prop and Roter smoothing / balancing computers, both Military and cival. Same thing on the EC135 and BO 105 European Helos.
It's somewhat critical to spacing from "whatever" it's getting the magnetic pulses from, to "count beans" for whatever. 0.095 to 0.125's usually worked well on aircraft applications, and for me, worked well on my Boss.
Stopped MOST of the hunting while @ a stop, and corrected virtually all anomilies while moving.
I've never seen a tolerance for the Boss application, for the Autometer Speedo, but mine was way too close, I "Dun-Dat" and the fault improved.
Hope this is helpful.