tiger :
May be applicable, may not be but I saw this in a CAA safety publication I received in the post yesterday.
SA350 - Cruise.
An investigation by the aircraft manufacturer into two reports of in-flight oscillation concluded that the problem was due to the Fuel Control Unit's ( FCU's ) lack of success in filtering drive train torsional mode. This 4Hz torsional mode causes velocity variations for which the FCU tries to compensate. However because the NTL ( free turbine ) and NG ( combustion chamber ) are in phase lag, the FCU doesn't solve the problem but sustains it. By modifying the flight conditions, the balance is broken and the phenomenon disappears.
It is noted - with no more stress on it that a pure mention - that this was a military Squirrel used for ab-initio training by the UK military.
Hope this helps. You modified the flight conditions - reduced power - and the problem disappeared, so this may be something after all.