Stepper motor step accuracy

Projects General Guidance
10

That power supply should work. However,

const int STEPS = 25; // 360 / stepAngle
// Initialize the Stepper class
Stepper myStepper(STEPS, dirA, dirB);
...
  myStepper.setSpeed(300);

STEPS is supposed to be the number of steps/revolution.
Can your stepper motor start from a standstill and rotate at 300 RPM? Try reducing the speed.