Mats, everytime someone posts with the problem you are describing, they get the response of "Coil problem". It MAY be this, BUT, it may equally be accumulated crud in the fuel system. When it happened to me this was the cause. And yes crud in the fuel system CAN result in 'firing on two intermittently" as you described (actually mine also became an intermittent single). It takes "Very" little crud to cause the problem. I'd suggest that you remove the float bowls to check. There is also a tiny little in-line fuel filter *inside* the fuel line, just behind the 't' piece, behind the carbs. Pull off the pipe & check this little filter. Virtually any crud there will result in your symptoms. I already had an extra fuel filter fitted near to the tap, but the very fine sediment that got through to the inline filter was enough to spoil things. FWIW, the easiest way to get the float bowls off, is to remove the carbs as one 'bank'. Leave the throttle cable attached as it's a b******d to re-fit at the carb end. Providing you don't alter any carb settings whilst doing this, you should be able to bolt them right back on, without need to 're-tune'.
If the mifiring happens when out on the road, one thing that helped me get home, was stopping at fuel stations & squirting a little compressed air up through the drain tubes on the carbs. This allowed the accumulated crud to be temporarily displaced, & the bike would run ok for a while longer, until the crud settled again.
Hope this helps,
Regards
Cuppa
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