Oh please do not change any gearbox for a torque indication - the only thing it can add to a torque problem is the Nr sensor, or a total GB massive malfunction - in which case, you would be writing this!
This surely sounds like an electrical problem, since the beginning, and I would bet it is a bad pin on a connector or bad shielding that is letting some stray signal into the torque indicating system.
To confirm: when you see the torque rising and falling, is there any change in the engine/gearbox noise? Any slight yaw motion? No consistent change in fuel flow and Ng/N1?
I would invite someone to take a roll of aluminum foil along, and wrap all the connectors with it, then duct tape them to keep the foil in place. If the wiggling goes away, then unwrap them in sequence, to see when it comes back. The factory guys know this stuff, they will get it.