Determing actual current demand from servos is trick without actual measurement is hard to do. It depends on the specific servo model, mechanical load on the servos, how much travel at any instance and of course if all are moving at the same time or not. It really is a common problem when people try to power more then a single servo from the Arduino's +5vdc source. I'm pretty sure having a external regulated +5 volts with enough current capacity to power the servo will fix your problem.
Lefty