On second look, that example code is clearly a demo designed to run a couple of DC motor in alternating directions, it's not designed to step a stepper. If that's what you're running, then a 500ms wobble is exactly what I'd expect to see. Just change the contents of loop() to do the pattern I provided above with 10ms between each step, using CW/CCW for the +/-.
The motor controllers have a current-sense pin which you will need to use to control your PWM to get the current right. I know nothing of the shield you're using but the current sense is almost certainly connected to a couple of Arduino analogue pins. Once you get the motor stepping reliably at low power, you will want to write a feedback loop (eg use the PID library?) to control your current properly; that is not an easy task.