Hello,
I have just find what was my problem.
2 swashplate servos are tired : the aileron servo and pitch servo.
there neutral position on load are not good.
the servo jerk at neutral position on rotor load and the neutral position is the pitch to hover.
this is why the heli has intermittent shaking at hovering because the neutral servo position has random jerk.

The overall heli shake is explain because if one servo change his position during hovering ( and the 2 other are rock solid) only one blade has a pitch change.
this mean that for a short moment the tracking is not good and the heli shake.

So I have tested the heli with very low and very high rotor speed to change the hover pitch rate. The heli is rock solid because the swashplate servos are working in a other position : a position that they don't jerk

and finally
I've tested the 3 swashplate servo at the workshop and I confirmed that the aileron and pitch servo have a worm potentiometer : the track is worm at neutral position.
Servos are Align 9X : about 100 flights.

So now know I have to update my swashplate servos.

Thanks for your help
best regards
Olivier