ATwin probl. Ignition, CDI, tacho?

[chiangsaen.peter]

I noticed on that my Atwin's rd07engine runs extr. uneven below 4000 rpm. Sounds like ignition problem. But then I also noticed the tacho needle jumping to and fro above 4/5000 revs. Piston shop suggest to switch CDI box with another Atwin driver.
I think there might be loose cable/connection?
Thanks
Peter