I found that at times the problem would disappear altogether and the bike would run fine. This lured me into a false sense of security, I'd go out on the bike and then it would drop a cylinder and the tacho would go insane. Changing the CDI's moved the problem to the other cylinder.

You could have a coil problem but if you look at the wiring diagram in detail it becomes obvious that the coils couldn't affect the tacho in that way. Maybe you have a combination of problems, best thing to do is sectionalise one component at a time.

Also check wiring remember you are working on an old bike which unless you bought it new you do not know the history of. I work in electronics and the simplest of things has caused me no end of problems.