S61 engaged in vertical reference operation in 200' hover.
Aircraft began short sharp vertical jumps -R/H pilot suspects coll. bounce but collective was immobile - friction not needed.
L/H Pic says controls are siezed.
Primary and auxiliary hydraulic caution lights begin flashing - both hyd. press. guages show very rapid on/off indications.
R/H pilot selects pri.hyd. switch off - controls still siezed - pri.hyd. selected back on and aux. hyd. switched off.
Aircraft stops vertical jumps and controls are operational again.
Aircraft returned to landing barge - artificial horizon(eyeball) was seen to be spinning very fast during return flight.
Suspected cause - rapid erroneous AFCS inputs overpowered hyd. systems.