My motors are clicking and making erratic noises?

Hello,
I just built a Prusa Mendel I2 from nwreprap, and the stepper motors will not work correctly. No matter what I do they always make jerky, inconsistent movements and strange, squealing sounds. Sometimes the motors don't move at all, they just make noise. I have adjusted the current up and down and still cannot resolve the problem. Any ideas? Thanks, icecats