Does the missfire occur in conjunction with the rev clock going haywire?
the cozzy rev clock is known to go a bit mad now and again, sometimes often spiking 7000 rpm when only doing 3000.
and the odd time a missfire can happen at the same time.had a problem like that a good while ago as well as several others on here.
the problem is appears to be earths, or more specifically the earth on the inlet manifold and or bad joints at the multy plug connections at the bulkhead.
that would be the first route to go, and see how you get on