Abstract
We investigate an unusual behavior of the parametric polariton scattering in a semiconductor microcavity (MC) under a strong cw resonant excitation: The maximum of the scattered signal above the threshold of stimulated parametric scattering does not shift along the microcavity lower polariton branch with the change of pump detuning or angle of incidence, but is fixed at normal direction. We show that such a behavior can be modelled numerically by a system of Maxwell and nonlinear Schrödinger equations for cavity polaritons and explained via the competition between the bistability of a driven nonlinear MC polariton and the instabilities of parametric polariton-polariton scattering.