Parts not animated moving and trembling

Hi,

We're having a bad time animating a character. The character is sitting on a chair, and only chest and arms are animated, the legs are not.

When we animate it in Unity, using Humanoid avatar, legs tremble and the entire model moves slightly in the air.

We have tried to enable and disable Root motion, to bake transforms into pose, to change model's pivot position... and nothing works. The only thing that is able to improve much the tremor is to switch the avatar from Humanoid to Generic, then the tremor practically disappears. Of course, we want to use Humanoid.

The pivot of this character between his feet on the ground, z forward and y upward. Tested in Unity pro 4.2 and 4.3.

Any idea how to fix it?