Is the Arduino PWM signal supposed to be stable? Because when we connect an RC servo and our PC scope we notice that the signal fluctuates a few microseconds which causes the motor to do continuous corrections. Is there add-on hardware with better PWM properties?
Are you talking about the hardware PWM or the signal produced by the Servo library? The hardware PWM at 490Hz is too fast for most 50Hz hobby servos.
Which Arduino, and how do you generate the PWM?