From what I know of this topic, which, admittedly isn't a lot, I do not think it is a wake problem.
Swirl in the rotor wake comes from two sources. Firstly, the air adheres to the blades which then drag it around. This gives a swirl velocity. Secondly, the root and tip vortices move down below the rotor plane in a helical path. Due to this angle, the rotational velocity has a component in the direction of rotor rotation. I grabbed 'Helicopter Performance, Stability and Control' by Ray Prouty and there on p.53 is some data from a US Army test. This showed that at a tenth of the rotor radius below the rotor plane, the average swirl velocity was at most 5ft/sec while the vertical was nearly ten times this. Also, the swirl effect reduces with distance below the rotor.
I remember seeing a film from the US somewhere (a Bob Tur film I think) which showed a crewman and stretcher at the end of a line spinning wildly. However, at the distance shown, I wouldn't have thought the wake could have an effect there.
I think the most probable cause is the many vibration sources in the machine itself. Lots from the rotor (flapping, lagging, damping problems, imbalance in the blades etc.) and also from the transmission, with all the rotating components.
As a weight on a line is, in effect, a pendulum, you would expect some oscillation. This could lead to swinging and/or rotation. In looking into this, such parameters are the weights suspended, the load mass distribution, line length and certainly the vibration in the machine itself.
Now, as said, I'm not an expert on this, so could have this wrong. So, if there are guns turning in my direction, put me right and help the bloke out! I would certainly expect there to be literature on this subject which would be invaluable to you. Plus, where are all the folk out there that have practical experience of this??
End of Rant2: The Rant Strikes Back