How far off path is it going? A few [edit: tens of thousandths, not ten thousanths] ten of thousandths, or much more?
Have you checked the tightness of the guide wheels against the vrails? (Shouldn't be tight per se, but should not allow the wheel or carriage to wiggle).
Have you checked the set screws on the stepper gears? Those have consistently given others trouble... verify that one of the set screws is on the flat spot of the stepper motor shaft, and that all set screws are tight.