el,
Without wanting to sound like a stuck record, the aircraft was dispatched in accordance with the MEL. If you are dispatching an aircraft in accordance with the MEL, you are not carrying out any troubleshooting. You are deferring that action to a time that is more reasonably practicable. If the MEL says go, then go. If there is any ambiguity, then write a better MEL.
But on the counter to that, this appears to be an intermittent defect. If it s an intermittent defect, then the TOWS would have failed numerous times as well. Why was this not spotted by the various flight crews before when carrying out a TOWS check?