I would think, while under a heavier load with more pitch the rotor head and blades are held firmer, and while just spinning free with less load it is being allowed to wonder? At a low head speed dampening becomes an issue, with less centrifugal force the blades have a greater tendancy to flutter. Maybe softer dampeners?
Some reading to help, http://www.cybercom.net/~copters/helo_aero.html
Get back to us regarding your resolution, I'm sure alot of people would be interested to hear the outcome.
I plan on building a semiscale UH-1 that I intend to run a low head speed on and less agresive cyclic, so I am one who is interested in any resolve for low head speed issues. Good Luck