jawzzz:
@MarkT I tried your suggestions. I used a moveTo() call when I receive the input from Serial but still get the jittery motion. Also, I cannot get the motor to move backward. I'm new to using the AccelStepper library. I've used the runToNewPosition() call with a Nema 17 motor and had no issue which is why I used it with the larger motor. I tried two versions of new code. The first is the Random example from the AccelStepper library.
You've checked the electrical connections to the motor, and that you've identified the two windings correctly?
Both windings are intact and the same resistance?
You have it set up for microstepping of course?