GPK1961 If not a synthetic oil problem would suspect a burr on the brass synchronizer ring,or a well worn 3rd gear. Might take off you pto cover and look to see if brass ring is too close to 3rd gear . Also if shift collar detents are kinda weak not putting enough tension on ring,slips in too easily. Should be able to feel that thru your stick. I guess you don't know if that is a new synchronizer ring?? My 2-ton 48 does that too pretty sure it's weak detents,gotta make me tear it out of there and put in some meaner detents !!