I had a similar prob years ago, i pulled that tranny 5-6 times and went through it and could never find anything wrong, then i started combing for any signs of a cause and i found the shift cable slightly out of adjustment going from 2-3rd, 1 or 2 turn adjustment and it was all good, go through each shift while the car is on jacks (engine off) and remove the cable from the lever and see if its clicking into each shift just right,