While working as a Aircrewman on a Bell 412 back in 1988' ( err... "Mobile-45")
we were conducting a 'High-line' transfer of a 'Ketch' in the Great Barrier Reef hovering at about 90 feet. The patient was being winched in a Strop/Hypo Strop and when he was at about 65-70 feet the Captain appeared to lose partial control of the aircraft. We began drifting left then right, climbing, descending etc not in an uncontrolled manner but!!!! things were not normal I continued winching in and con the aircraft as best I could (which was not having any effect) the Captain did not respond to any commands, comments or swearing? The Co-pilot eventually took over and we settled down at about 20 feet, approx 150-200 feet (rotor disk) from the Ketch. We recovered the Rescue-Crewman who had the high-line pulled out of his hands (he jumped in and swam over to us when signalled. The Captain was stood down from flying duties, pending several tests. He had the tests and resumed full flying duties subject to flying two pilot ops.
We were all briefed that he suffered from what they called at the time 'Rotor Flicker'. He had had a similar incident during his time in the RAAF flying Huey's.
Cheers
Clearance.