A strange shift in the x-axis direction ... electronics or software ??

Hi Reprappers ... I built a new 3D printer "SimpleBot V1.0" you can find it here : [sessem13.tumblr.com]
now I'm having serious calibration problems !!
the first stupid thing is that big shift or wrong travel in the x-axis direction... you can see that in the attached pictures :

I hope you have a cure for my problem ??

I'd bet you have your current set so high that the motor drivers are cutting out. Second guess is that the set screws on the stepper shaft are not tight.