Dead zone in steering

What do you mean by "dead zone"? Do you lose power assist? That would be very unusual. Does it get tight in one area? The better suspect for that is a binding universal joint on the steering shaft, under the dash. To verify that, first inspect them for signs of rust-colored powder around them. Next, you might be able to feel them binding as you turn the steering wheel, but to know for sure, you have to disconnect the steering shaft from the rack and pinion assembly. At that point you have to be very careful to not turn the steering wheel beyond its normal limits. Start with the tires straight ahead. Disconnect the steering shaft, then never turn the steering wheel more than one revolution to the right or to the left, and be absolutely certain to return it to where you started. There is a "clock spring" under the steering wheel for the air bag. That is a wound-up ribbon cable in a plastic housing. If you turn the steering wheel one way more than two revolutions, that cable will become tight and tear off. Of you go the other way two revolutions, the cable will unwind too far and fold over on itself. It will break if that is done repeatedly.

It is also possible to have binding upper strut mounts, but the symptoms are typically not what you described. To identify this type of problem, jack the front end off the ground, then push the tire left and right. If you still feel the tight spot, the upper mounts are not the cause. The car needs to be supported so the suspension is hanging down freely. That removes the weight from the strut mounts and will let them turn freely if they were binding.

That CAN be caused by a loose steering gear. One clue is you'll hear the knocking of it moving around when you work the steering wheel back and forth vigorously. Also, you'll see it move if you crawl underneath and watch it while a helper works the steering wheel.

This can also be caused by badly-worn tie rod ends, ... Usually the outers. Also watch the universal joints in the steering shaft, under the dash, and watch for any movement of the input shaft to the steering gear that isn't met with matching movement of the inner tie rod ends. Play in the spool valve can cause that, but you'll almost always have a severe power steering fluid leak before the valve gets that bad.

There is an adjustment that can cause this symptom if it is not adjusted correctly, but it doesn't change on its own. When the play shows up later, it is due to worn bushings or a worn spool valve. Repair parts are now available for some steering gears but it is not practical to repair one unless you're emotionally-involved with it. The cost for rebuilt gears has come don so much that it doesn't pay to try to fix one.