An anti-stall tool in an oil well drilling assembly that controls reciprocation of the drill bit by a controller that alters weight-on-bit (WOB) depending upon measured downhole pressure or torque at the downhole motor. The controller receives preset high and low working pressure limits for the downhole motor and keeps the drill bit rotating by maintaining WOB during normal drilling operations, increasing WOB if sensed working pressure indicates that drill bit loading or torque is undesirably low, and reversing WOB by retracting the drill bit if excessive working pressure or torque is sensed.