that lil box is usually where all the shifting problems happen - there's a pawl that
catches slots - the pawl and slots get worn and the movement gets trapped between gears - you need to get that box right before anything else [open it up and clean it and regrease, make sure the spring is there], then the rod length is adjusted by threading the top knuckle up or down ... and make sure that coupling is really tight, even when the bolts are in, the rod may twist in the coupling
Bill