You can get a variation in the torque gauge indication if the oil system to the gauge has air in it. The remedy is simply to get the system bled.
However, it sounds like what you describe is a real variation in torque which is different and certainly worthy of investigation. With a change in torque indication coincident with a change in Nr/N2, the most likely thing is a real change in torque - not just a torque indicaton.
What have your engineers said? - You should get them involved pdq IMHO