Hey guys, thanks for the input.
Have just had loan (overhauled) serviceable mags fitted and the problem is still there, it makes no difference if the governor is switched on or off. Are now thinking of looking at fuel servo or flow divider!
Just to add also that ship during normal flight is fine, when the engine RPM is dropped below 80% the fault occurs again, you can imagine what that was like recovering from an auto-rotation! So the fault is only there during that narrow power band between 80-95%.