[Bigdav160]

Your description sounds like excessive clearance in the box. Likely not enough preload on the worm gear. That is set by a turning torque measurement like setting preload on pinion bearings in the rear axle. You could remove the rag joint, loosen the large lock nut and using a pin spanner tighten the adjuster plug slightly. Slightly meaning not more than a 1/8" at a time