Inertial Units
'The IR units are self-contained: they are calibrated sometime way back when and that’s it for the remainder of the flight. So when they start complaining, it is either a system fault or you are already out of control and moving them around more than they judge appropriate.'
These are strapdown systems which integrate up rates and accelerations. If the dynamics exceed certain limits the attitude reference will diverge as the integration errors mount up and effectively cause cross channel coupling as the sensor inputs are incorrectly resolved. However do you know that air data isn't used in some damping mode via clever kalman filtering or integrity/sanity cross-checking ?
Any significant to apparent lack of faulting from other systems eg engines ?