Erratic stepper question.

I have recently finished assemblying my 3 axis 5amp stepper driver
from Camtronics and my homemade power supply. I downloaded the CNCPRO
demo and ran each stepper motor in air. I noticed that at certain
speeds the motors shuttered, especially at very low speeds. I also
loaded the example 3D arc G code file that came with the software,
when I tried to run it the motor shuttered a couple of times at low
speed. As the speed ramped up sometimes the motor would take off
really fast in the wrong direction. I am running in half step mode
and I believe all of my software and driver configurations are setup
correct. Could anyone please tell me if I'm missing something.

The motor details:

Superior Electric
Slo-Syn Synchronous/Stepping Motor
Type= M091-FD-426
150oz.in
200 Steps
2.6V 3.1A DC
6 wire

I bought these motors at C and H Sales.

Thank You,

Jason