A steady-state dependence of slip velocity on shear stress at polymer/solid surfaces is derived from a molecular model that includes effects of drag on polymer chains, disentanglement, and detachment and reattachment of chains from the solid surface. The dependence includes two turning points and thus a region where slip velocity is a multivalued function of shear stress. This behavior is consistent with experimental observations of abrupt and hysteretic transitions between regimes of small and large slip. Parameters involved in the model are estimated and a reasonable comparison with experiment is obtained.