What PC are you driving it with? is it your normal desktop machine or one dedicated to running mach3 or simular.
I had a simular problem where nomatter how i connected the motor i could not stop it jumping around, it turnd out to be somthing with the isa paralell port in that the pulses were not all of the same time and thus made the motor jump in all directions.
I have lernt never to test stepper motors with a random machine running Mach3.
Roger