Lost power at 55mph

[Redleader]

So I went for a ride last Sunday I had been riding for about 30 mins so everything was nice and warm. I was doing about 55mph the throttle was at about 90%. Then all of the sudden it was like it was slowly losing power, I dropped the clutch rolled off then on the throttle came back to life. It did it again about 2 mins later so I turned around went home. Everything seemed ok it had about 75% of the tank of fuel. But earlier that week I had to turn up the idle to about 1250 or it felt like it was barley running. I did the valve lash about 900 miles ago and aside from this it had been running pretty well. According to the manual you only adjust every 3000km or 1800 miles i German spec’d