rfs56trk is exactly right.Closely inspect the attachment points of the shift linkage for looseness.If the shift detents are not fully positioned they will work to move the thing out of the effected gear.position the DIDCONNECTED shift arm in first gear then see if the shift rods are going to bring it to that point.every time.No looseness allowed.There was some hardware in the rod terminations that can not be replaced with common washer and cotter key.Of course there is a procedure for proper alignment of all the linkage and someone here will probably supply.
