The bistability problem, common to scanning microscopes employing lateral dithering of the probe for image formation (i.e., shear force microscope) or probe-sample distance control (i.e., near-field optical microscope) is shown to stem from the two nearly degenerate vibration degrees of freedom possessed by a laterally dithered fiber. Controlling the fiber vibration direction by means of a four-sectioned piezo was found to be a simple and effective solution of the problem. An image of a microtubule is presented to demonstrate the improved imaging ability. ©1997 American Institute of Physics.