We calculate polar motion in models of 3-D spherical mantle convection at Rayleigh numbers up to 108 which include internal heating, radial viscosity variations, and an endothermic phase change. Isoviscous models yield rapid polar motion of order 3¿/Myr, but a factor of 30 increase in viscosity with depth reduces the rate of polar motion to about 0.5¿/Myr due to stabilization of the large-scale pattern of convection. Avalanching due to an endothermic phase change causes pulsating inertial interchange polar excursions of order 80--110¿ and of duration 20--70 Myr. A layered viscosity model with an endothermic phase change yields only one inertial interchange event in 600 million years. These models show that the slow observed rate of post-Paleozoic true polar wander is not incompatible with higher rates inferred for earlier times. ¿ 1999 American Geophysical Union