We study the coupled Wong Yang-Mills equations for both $U(1)$ and $SU(2)$ gauge groups and anisotropic particle momentum distributions numerically on a lattice. For weak fields with initial energy density much smaller than that of the particles we confirm the existence of plasma instabilities and exponential growth of the fields which has been discussed previously. Also, the $SU(2)$ case is qualitatively similar to $U(1)$, and we do find strong ``abelianization'' of the non-Abelian fields during the period of exponential growth. However, the effect nearly disappears when the fields are strong. This is because of the very rapid isotropization of the particle momenta by deflection in a strong field on time scales comparable to that for the development of Yang-Mills instabilities.