It sounds like a broken or worn tooth on one of the gears inside the steering box or a worn case.
Take the four bolts off the top of the steering box. The column, shaft, and steering wheel will all come off together. Be careful with the gasket, it needs to be there even if it's ripped(until a replacement is found).
Inside the steering box, lift out the reduction gear.
There are two pins sticking up out of the box. They should both be held tightly(pressed in) into the steering box.
You can inspect the reduction gear, and the steering shaft gear for worn, or broken teeth. You can also see the teeth on the rack at this point.