I think that the problem isn't the power source of the motor, because I didn't connect it to Arduino to check the if it was a problem of the motor. But when I cheked the PWM output without the motor conneceted, I have noticed that there are a problem with the PWM generation.
Also I have tested with a servo of 6V conected with a 7806 and a 5V servo conected to arduino. And the same errors...I think it's the function servo, but I am not sure.