I am trying to use a 555 timer to create the PWM signal for a servo motor. I found and constructed the circuit in the attached image. It does work as advertised, and I can adjust the servo's position via the potentiometer (labeled VR1 in the circuit). However, when outside torque is applied to the servo, it does a poor job holding its position. I can turn it several degrees at which point it jitters.
I know this is not a power supply problem because using the same setup and a signal generated by a microcontroller, the servo can hold its position perfectly against any torque I can apply. So I am guessing there is some noise generated by the servo under load that is causing the timer to misfire. If this might actually be the problem, where that noise would be, and how to suppress it I do not know. So, my question is: how can I modify this circuit so the servo has more stable performance under load?