I know exactly what you're talking about. I've had to talk with the our machine shop a couple of times about that joint there--They try to round the edges on the aluminum parts to break sharp corners--but if they do it too aggressively they can introduce a slight slope there on the large X/Z carriage plate--which can cause what you're seeing.
However--we test run the tools in all axes at 6 inches per second (the faster the motors run, the lower power the movement and the more prone to stalling). So I'm confident that we can get this back into adjustment. I think you want to make sure that the nut bracket (the little T shaped thing) is on as flat a footing as possible. The hole through which the two screws pass is slightly oversized, allowing you to shift the bracket a little bit before tightening down. Try moving the X axis so that the bracket is pushed as far into the X/Z carriage as possible, away from the rounded edge of the plate.
Depending on when your tool was built, it may have a little hole in the right side wall, you can pass a 4mm wrench through there and turn the motor manually--to get an idea of how this works, try it with the Y axis motor, much easier to see there. Do this with the tool powered off--and you can get a good feel for when the binding occurs--which will be where you want to make your adjustments.