Transmission work is all about clearances and tolerances in the + or - .001" range and requires patience sorting it all out. Best to find a quality shop that has all of the proper thrust washers and spacers on hand if you're going to have to pursue that route, preferably with some revered old geezer that scares the rest of the mechanics. You have to pass through a momentary neutral to shift anyway as the gears slide back and forth on their respective shafts in the tranny since you can't begin engaging one while in contact with the another, it's the dog postion that places them in the correct alignment for each gear and that's the fine tuning that you're likely lacking...sounds like you might have started on the "wide" side new and normal wear has moved you past optimal location.
