Thanks for the answer, but I fear I didn't explain the indications well enough.
The torque will fluctuate, but in a very rythmic way, from about 92ish up to 100 and down again, the frequency not changing for the duration. And every time it increases there is an audible change in the noise from engine/gearbox and associated yaw..so it is not just the gauge. The frequency is about 2 per second. While this is going on the N1 is stable, but N2/NR fluctuates about 2% with the torque..same frequency. Then after a while it settles down again and both indications are back to normal and steady (as steady as they get).
Also happened once in a climb, 80% TQ and 60mph, steady climb. Torque was 80-90, same frequency and sound, yaw being less because of the tail being effective, but still noticable.
Total duration is around 10sec+ before it goes away.
Ken