Servo jitter with Arduino
According to (MG996R)'s datasheet
You must connect a 6 volts - 2.5 amps power supply for it's best performance (max. Torque)
Maximum voltage of this servomotor is 6 volts
And be careful, exceeding that voltage can be cause of some damage to it's control board
Try 6 volts 2.5 amps power supply and if doesn't work maybe your servomotor's control-board is damaged by over-voltage.