First, why are you not using the servo library to control your continuous rotation servos? The servo library will handle the automatic updating of the last valid position command sent to the servo at the frequency that the servo is designed to work at, around 50 updates a second. Your manual method is dealing with the width of the control signal sent to the servo but not refreshing the value at the rate the servo requires.
Use the servo library, you will have much better control.
Lefty