Failing Autorotation Bearing
Engine Sneezing

The one-way autorotation bearing apparently is not as strong as it should be. The local shopman told that he has sold already 3 new bearings, while he can't remember anyone asking those for hirobo.
Anyway, the behaviour of my broken autorotation bearing showed to be very weird. It seems to slip for short moments, causing the engine to speed up instantaneously, followed by an even abrupt return to the old RPM after half a second or so. The RPM increase is dramatic, I estimate it to be 50%, so you may be hovering at 14000rpm and suddenly your engine runs at 20000 for a short while! It sounds like your engine is 'sneezing'.
I have read somewhere that erratic behaviour of this bearing can also be caused by overtightening the jesus bolt. Overtightening this is not a good idea anyway, as the head of these bolts is notorious for breaking off.
It seems very hard to see when the bearing is worn out. The pictures below show the clutch shaft. Apart from a little brown residue, the surface still seems perfectly smooth, so if there is wear it must be in the micrometer range... It can also be that the bearing itself is worn out, maybe the needle springs get weak or so? Anyway, this is even harder to check or make photos of... I had to replace the bearing in the third year, after about 300 flights, but looking back I wonder whether problems I had in the second year were already pointing to this bearing..