help to fix tachometer problem

the tachometer in my 128 coupe it's electronic and takes the signal from negative tip of the ignition coil; since few days, it starts working normally but after few km of riding, the needle goes crazy bouncing from 4k to max, beating hard on the stop all the time; if i stop the car for few minutes, it starts again working normal, but after a short time, again  with the crazy dance.

so it looks like related with something that become hot..

i checked wire connections and it look all good, so i suspect some electronic components (capacitor\restistors? ) inside the instrument shell..any experience or electronic knowledge on this?