Guys,
After experimenting for the last week, I have found the explanation and the reasonable solution to the problem.
First, let me restate the problem the way I see it now: when ANY collet style propeller adapter is used, the combined effect of torquing the prop's mounting nut AND the adapter's shaft bending stiffness AND co-linearity of the collet under preload, may cause the prop's blades to deviate from the desired plane of rotation. This behavior is random, depends on the angular position of ALL the pieces that are squeezed and may or may not manifest itself. If it does, one of the 3 blade propeller's blades will be bent out of the ideal plane of the prop. rotation more than 0.002" that can be measured by a good quality metal caliper.
Please see the attached. In my case, the addition of the hard rubber washer, separating the prop AND the aluminum prop. support ring solved the problem.
All the 3 blade carbon composite props I have (see the attachment, please) rotate now true and well.
By "true and well" I mean that the blades tips stay within 0.002" from the theoretical plane of rotation.
Regards and Happy Flying,
M