Ok well every thing looks ok wiring wise but I would dump the PC/Mac setup and use dedicated PC setup.

Set the drives up using the settings Eddy provided.

Then as a little test using this PC/Mac setup try this. Go to MDI screen and type G0 x100 and see how it moves. (G0 is G (zero) not "O")
If you still have harsh movement then I would try another PC setup before doing anything else.