The stepper library does not know that you have an a4983. It is toggling the pins alternatly, thinking there is a motor. As one of the pins is the "dir" pin on the chip, your motor just jiggles to and fro.
Control your pins directly. Leave the pin going to the "dir" high and then alternate the the "step" pin up and down with a delay.
(I'm sure someone has made a library, but it is just a 3 line program to control the motor through the A4983.)