Servo motors twitching , arduino Mega
I suppose that the twitching is caused because each servo draws high current from your battery/power supply for a short period of time (average current is low, but peak current is high). Those high current spikes upset the smooth supply voltage. Try to add a decoupling capacitor in your circuit. (2200 uF let's say). I saw some solutions with a metal core wrapped around the wires of the servo for filtering. Also you should try to put some capacitors across the motor terminals ( .01 uF) and a high value resistor between signal wire and ground. That should eliminate the random movement.
At initial power-up the twitching is not totally eliminated, I think that depends on your code.