Worn joints.
It all depends on how much slop there is, how well it shifts. If there is slop or sticking, you will probably have rotational shaft "bushing" wear and you will have pivot point wear. Some of it may be pot metal (hard to add metal to fix, hard to weld) and some will be rods/levers that may have wallowed out/elongated holes. These holes can be welded and redrilled, bushings replaced and the worn rods replaced/welded. There may be shafts to re-new also.
There are a number of mechanical features involved all the way to the tranny levers. A little wear in each can add up to substantial lost movement or wiggleization. Observation will tell you what to do. I don't know if there are any bearings or not. If there are, that's another fix point.
