Shaky servos

I am building a robot arm with a SCARA style. I am using HiTec 805mg servos for the base and elbow and HiTec 685mm for other parts. The servos in the elbow and base seem to shake when standing. I am controlling all servos from arduino mega with a sensor shield on it. Why do the servos shake? Using the Servo knob code.

This might be the slight jitter introduced to the Servo library by other interrupts (in particular the timer0 overflow interrupt that drives millis() and delay())

Most likely insufficient power supply to hold the servo in position under load.

I am using 7.2V 3000Ma battery pack. What should I use instead? Do I just need to fully charge it?

Does the servo shake when not under load? If it does, then there might be some issues. If it does not shake when not under load, then you may need a larger servo or different arm design.