Luca hi mate,
Thanks for your input. Mine is a diesel sorry should have mentioned that But i have seen what you described in many cars many years back, both ford and holden 6 cyl engines used to do it. I could see the spark running down the coil at night, looked like a lightening flash.
Anyway, I am pretty sure I will find the cause, most replies from other forums suggest its more common than we think. Most common cause being a dry solder or bad earth behind the cluster.
I will run a earth wire from the cluster to the battery first and try that, if that fails then i will take the tacho out and check all solder joints.
I am sure one of those will sort it out.
Thanks again for the feedback guys.
Hope to see you out there.