I have a similar problem withmy MC18. I have just had all the carbs apart and made sure they were clean, put it back together, put in new plugs and it runs great...for about 20 mins, then one cylinder goes down, so I take a plug out of my mc28 the it starts to work again fine until the other plug stops firing, what can cause a plug to fail (they still look new and clean).

Incidently I have done a compression test on both cylinders are fine.