Drweir, make sure you haven't swapped your x and z endstops. Engage them manually when setting up your printer. Understand that the printer should move Z axis first, then X and finally Y. When it is moving the build plate down, try to depress one of the other endstops (X) to see if you have it miswired. Further, make sure your grub screws are all tightened and none of the pulleys are slipping.