Just listening inattentively in the back benches here.......
But the topic reminded me of a tour from hell in a remote area with a newly acquired bucking bronc of an L3.......
First of all is the torque fluction REAL and ACTUAL, that is; felt in the seat of the pants by way of yaw, heard, and correlated by a bouncing nr gauge?
Or is it just the torque gauge itself bobbing up and down 10 percent or so?
If you think it's ACTUAL. Start with the easy stuff mentioned so far PTG, FCU. But on the c30 don't rule out the combustion liner can. Yup, the combustion liner. Had very similar actual torque bob with a beat of about 40-50 beats a min.
Changed out parts willy nilly and low and behold it was the combustion can had shifted minutely causing a surging flame pattern.
If it's just the torque gauge and all the air has been bled from the system repeatedly then hate to say could be a gearbox issue with torquemeter system hydraulic issues.
Good luck