And the other weird thing that when I unplug the radio modul and the Servo is still under power by the ESC BEC, the shaking stops. So I don't know if the problem is voltage fluctuation or signal interference.

If voltage fluctuation is the problem, how can I solve it? I know I should solder a capacitor between the ground and VCC of the ESC BEC, but this would require cutting open the cable's insulation, which I am reluctant to do.