Fluctuating rev counter

The rev counter on the clock seems to work when it likes. You can see it fluctuating really badly when it does work.

What could this problem be?
Anyone else had any similar experiences?

Regards
vw_tees

Earth wire back of cam cover ok?
Wires ok at coil?

I had the same issue on a Mk2. The rev counter used to go all over the place. I tried new instrument pod and even took it to a supposed auto electrician. In the end it was poor HT leads to the plugs.