# DC servo motor shake/vibrate too much

I have a servo motor (details are here). I wrote a code in Arduino to rotate it in some defined angles and position. Code is given below:

void setup() {
// put your setup code here, to run once:
Serial.begin(9600);
Serial1.begin(9600);
Serial1.println("M40");
}

void loop() {
// put your main code here, to run repeatedly:
if (Serial.available() > 0)
{
}
if (Serial1.available() > 0)
{
}
}


Initially, the motor rotates fine and gave me feedback also. But, since last week whenever I gave a command to rotate motor it starts shaking/vibrating. I don't know why? I used same circuit, same power source etc. as I used earlier. I contacted company's people but they are also not getting what the problem is? Using multimeter, I checked voltage and found out 12.9 V which is within the range.

Can anybody help to find out the reason behind these? I have 2 motors and both are showing this behavior.

Thanks.