I experienced a similar issue on my mc18 which had two causes.

1. One of the spark plug caps had a big crack in it which was effecting spark to one of the cylinders.
2. The crank center seal was stuffed.

If your bike has developed this issue suddenly than it could be #1 (if you have ruled out RC valves etc). If it has been sitting for a time, then it could be #2 which is a lot more expensive to rectify.

Good luck.