Jo, not so sure it will be a coil, i would expect a missfire or rough running under load or once the coil warms up.
i suspect as has been said a poor earth on the loom / frame.
remove the left hand mid panel and clean up the earth which goes next to the front coil, also check the earth strap from the battery to the frame.
the tacho feed comes straight off the CDI so thats also a possibility. but with your bike history i still rest my money on earth

I'm meeting up with Sam (billing cbr) tomorrow and he's bringing a spare CDI so we're ognna swap them and see if that works.
The earths should all be ok, I checked and cleaned them all when I had the last rectifier failure.

CRM wrote:Jo, not so sure it will be a coil, i would expect a missfire or rough running under load or once the coil warms up.
i suspect as has been said a poor earth on the loom / frame.
remove the left hand mid panel and clean up the earth which goes next to the front coil, also check the earth strap from the battery to the frame.
the tacho feed comes straight off the CDI so thats also a possibility. but with your bike history i still rest my money on earth

Yep.. sounds like a bad earth or a dodgy wire somewhere. Perhaps the tacho feed wire has a poor connection (i.e. a partial break in it) thats causing a bad signal, shame you dont' have another tacho to swap.. would isolate tacho unit itself.

Hopefully the ignition box hasn't been fried

Tacho signal comes from CDI unit, if bike runs ok then tacho drive section of CDI might be dud. As suggested by BillingCBR, if you swap instruments it will eliminate the tacho & you can check the tacho wiring connection at the same time. The tacho signal, a single separate plug & socket in the group of connectors located at the rear of the instrument cluster, might have come loose.

it was the rev counter in the end!

cool so thats fixed then.

I like a happy ending

thanks guys, I'm very happy as Rex my car is currently at the body shop undergoing arch and sill surgery so need my bike to get out of this middle of nowhere village. Sam (billing cbr) had a helping hand in it. We found out the clocks I had aren't the right ones, luckily he has a spare set. Wrong clocks have yet to be identified, they are very similar to the NC30 but have the wrong bracket and red lines lower. Pics to follow...

A pic of the face of the clocks is on my fb Jo :)