The S70A-9 has always had an issue with Np/Nr droop, due to the analog ECUs on the engines. This was identified as a problem back in 1989, and the proposed fix from GE was to retrofit the fleet with digital ECUs. The decision was made by Army not to fit the digital ECU at this time. The analog ECU just cant cope with a rapid and significant power demand.
The later S70's, eg China Hawk and Turkish Hawk were (I think) fitted with digital ECUs...