Electrohydraulic turbine controller (Bosch Rexroth STW high response prop valve) was found to be hunting over a range of almost 10% which resulted in a load throw off of almost 40-60 MW suddenly. Trend shows that the actual position of the piston driven by the prop valve started hunting first followed by load throw off. Checked for airlock and vented out some oil. But again in sometime started responding the same way. Need some help in this regard.