Unless the copper flash checkball has been removed from the valvebody, having the TV cable disconnected should cause late harsh shifts because it causes the TV boost valve to default to high line pressure. It should not cause slipping in 3rd. Most likely cause of this problem is damaged 3-4 clutches. It could also be a rolled or cut 3-4 piston lip seal or a cut or damaged sealing ring on the turbine shaft.
It is also possible that the 3rd accumulator checkball is not sealing. This isn't so much an issue with the 2-4 servo assembly. It is however related since the 2nd servo is also the 3rd accumulator. The 3rd accumulator checkball is open in 2nd gear and allows the 2nd servo to move and engage the 2-4 band. When the unit shifts to 3rd gear, pressure is applied to the release side, or 3rd accumulator side of the 2nd servo. This closes the 3rd accumulator checkball and forces the servo back releasing the 2-4 band. This same pressure also applies the 3-4 clutch.
Most likely though, it's just burned up 3-4 clutches. Huge failure item.