I would guess the spring in the detent for that shifting rod is bound up or broke.I would also expect it to jump out of 5th gear also since that is on the same shift rod.I just did a restore on a 70 and had that area all apart.Can you feel a difference in the detenting from the gears on the left side of the shift pattern, and the gears on the right side?All the gears on left side use same shift rod,and gears on right side use a different one.  Good luck! Dale H in PA