I was riding into work this morning when my bike started to misbehave. I was overtaking someone, doing about 11K RPM at 2/3 throttle when I suddenly had to back off then get back on the gas. Only there was no getting back on the gas. It's coughing and spluttering and generally not producing any power. It's ok up to about 1/2 throttle under 4.5K/5K RPM, but then it just coughs and splutters. It free revs OK (I don't do that beyond 9K RPM, but it was fine to there). Any throttle setting over 5K RPM induces coughs. Any ideas?