Electronic Speedo issue? Help

Hi all, Got an Autometer electronic speedo hooked to my Trimatic in the torana. When I calibrated it, it was working but was bouncing around so you couldn’t read it that well so I calibrated it again and now it doesn’t do anything??? Question 1: Are these known to not read steady?? Question 2: Any ideas what is wrong with it? I originally calibrated the distance using the Miles calculation (2mile I think it was) then I tried doing it in km’s (2km) Gauge is both miles and Km’s. Any ideas?