I had a similar occasional misfire under load and initially thought it was one of the coils was breaking down.
However, while I had it in bits recently to change the belts I decided to check the plugs (that I supplied to the garage to fit when I 1st bought it)
And guess what....I think thats all the did because I could have flown a Jumbo Jet through the gaps!!
So, set gaps to spec, refitted and went out for a ride, feels and pulls like a different bike now!!
Its worth a check, as if the plug gap is too big the spark can fail when under load..