How does my 727 trans lock up

Hey guys I have a 727 transmission that locks up automatically in 3rd gear. In the past I have run the 46rh / A518 transmissions which you have to supply power and an earth to get them to lock up via a solenoid.
However this one just locks up on its own and only in Top gear. Does anyone know if it's the converter that losck it up or the valve body. I'm chasing a vibration and would like to try another valve body but I only have one out of a non lock up 727 to try.