I purchased a used HD tachometer from a friend of mine. I had to make my own wiring harness and I hooked up the ground and the light/power wire. I coiled up the trigger wire at the ECM and had the dealer make that connection to the #3 terminal at the ECM. The tach seemed to work ok but I never really rolled it on since I had it hooked up. Today I started to put it thru the gears and opened it up. As soon as the tach reached approx 3500 rpm the needle started jumping + - 1000 rpm's. If I were to throttle up at about 1/2 throttle the needle is fairly smooth thru that same range. Also when stopped and revving the motor it seems fine also. I checked all the connections and they seem tight and OK. The only thing that comes to mind now is possibly a bad tach or could it be RF getting into the trigger wire under hard 100% throttle and load. I did use a solid (no strand) wire for the trigger wire. I don't know if I should have used stranded wire for that. Does anyone have any thoughts?? This is on an 07 fxstb. Again normal riding seems to be perfect on the tach.