Punto mark1 speedo goes haywire. Flashes up to 120 mph, down to 20mph (where it sticks) and then works OK after a while until it does it again.

Thought I had diagnosed the problem as the speedo sender unit. New unit solved the problem for all of 20 miles and now its doing it again.

Any ideas or experience please

Are these cable driven? Sounds like a cable problem to me