In my experience this sounds like the CDI and was is exactly how mine first started behaving. Mine would always play up on one cylinder swapping the CDI's over moved the fault to the other cylinder and make the tacho/rev counter go nuts. I guess it could also be a fuel blockage but seems unlikely.
I did manage to repair mine but wouldn't recommend it unless you have alot of time to dig the potting out. I believe they are still available new however, I sourced a new one in the uk anyway but got the old one fixed whilst waiting.