Thanks again Cflight for the answer.
I just have a look at the heli aero and I really don't understand (In my case) why tired main bearings will do intermittent shaking at high rotor speed, and no shaking at low rotor speed.

My wish is to fly my heli with high rotor speed with no intermittent shaking.

I will change the main bearings as soon as possible and tell you if it is the solution but really (may be I'm stupid) I don't understand why new main bearing will help me.

Regards
Olivier