Electronic – Arduino Motor Shield for 12v stepper motor

I'm making a basic rig for timelapse photography using arduino.
It is a panning system for a DSLR camera, so I went with the motor shield from Arduino with the L298 chip and a 12V, 1.7A stepper motor. Before I could power the motor shield externally I noticed that the motor was running fine off of the 5V USB supplied to the Arduino board. It was able to fully move the DSLR around at different speeds and hold it in place.

The problem arose when it was asked to do a single step at a time (I assume setting up any speed here will do). I noticed that not all steps are equal. Sometimes, there would 2 small steps followed by a larger step or even the other way around. From my understanding, stepper motors have fixed step size, and either it will fully do it (provided with enough power), or it will fail, is that correct? I thought it will fully overcome the inertia, and not get stuck half way, and then work fine other times.

So is the motor/5v power or my code is at fault here? Providing external power would require me to cut a trace on the shield so I'm holding out on that till I'm sure. (also having a single power supply would be nice!)

Here is the generic code I'm using:

#include <Stepper.h>

 const int stepsPerRevolution = 1024;
 Stepper myStepper(stepsPerRevolution, 12,13);     

// give the motor control pins names:
 const int pwmA = 3;
 const int pwmB = 11;
 const int brakeA = 9;
 const int brakeB = 8;
 const int dirA = 12;
 const int dirB = 13;

 int times = 120;//420
void setup() {
 Serial.begin(9600);
 // set the PWM and brake pins so that the direction pins  // can be used to control the motor:
pinMode(pwmA, OUTPUT);
pinMode(pwmB, OUTPUT);
pinMode(brakeA, OUTPUT);
pinMode(brakeB, OUTPUT);
digitalWrite(pwmA, HIGH);
digitalWrite(pwmB, HIGH);
digitalWrite(brakeA, LOW);
digitalWrite(brakeB, LOW);

// initialize the serial port:
Serial.begin(9600);
// set the motor speed (for multiple steps only):
myStepper.setSpeed(2);
}

void loop() {
  while(times>0){
    myStepper.step(1);
    Serial.println(times);
    times--;
    delay(12000);
   }

  }