Large scale one-dimensional hybrid simulations with resistive electrons have been carried out of a quasi-parallel (&thgr;Bn=30¿) high Mach number collisionless shock. The shock initially appears stable, but then exhibits cyclic behavior. For the magnetic field the cycle consists of a period when the transition from upstream to downstream is steep and well defined, followed by a period when the shock transition is extended and perturbed. This cyclic shock solution results from upstream perturbations, caused by backstreaming gyrating ions, convecting into the shock. The cyclic re-formation of a sharp shock transition can allow ions, at one time upstream because of reflection or leakage, to contribute to the shock thermalization. The simulations are simplistic in a number of ways for the quasi-parallel shock, but their results suggest a model which may explain qualitatively several features of observations. ¿ 1989 American Geophysical Union