I asked earlier, and I think this answers my question - when your student box loses control of the plane, you're saying you can't reclaim control of the plane by releasing the trainer switch.

Yes, it sounds like you have a bad trainer cord. Something really fishy, though, is responsible for not being able to release the trainer switch and take control of the plane.

Does the instructor transmitter you are using have a good range check? Any intermittents? This does not sound like an open in the cord. However...

One thing that occurs to me is the power circuit. The cord provides power from your instructor's transmitter battery to your student box; the student box uses the power to create the signals for your transmitter to broadcast. In the event that you are shorting the battery wires together when you twist the cord, that would (1) kill power to the student box, and (2) short your transmitter battery, killing the available voltage to your transmitter. In that situation, no signal will be generated or broadcast, in either trainer switch position.

If the above is the root cause, you can confirm with an ohmmeter and a little cord twisting. Actually, I suppose a shorted pair that carries the signal could likewise interrupt control.

Good luck,
Dave Olson