I'm going to bet it's the bearings in the steering box. There should be an upper and lower on the steering wheel shaft. They act like thrust bearings for the shaft and depending on which way the wheel is turned, the upper or lower takes the load for the gear engagement in the box. When they go, it allows the sector gear and steering shaft nut to slip teeth or not engage. Check the box out and if it is bearings, they are probably available from any of the online Hinomoto parts dealers. It's a cheap and easy fix once you get the steering shaft off the box. Is there any up/down play in the steering shaft when you turn the wheel? If so, it is almost certainly one of those bearings.
