Cartesian displacements yield usually a higher accuracy than Z-matrix displacements because in the former case cancellation of numerical integration errors between the different geometries is (almost always) larger.
If Z-matrix coordinates are used as the displacement variables, then make sure that no bond angles of 180 (or zero) degrees are among them. They will very probably be treated incorrectly. If your molecule has such bond angles, use dummies to redefine the coordinates or use Cartesian displacements.
