Yes sebaslive, my kit is in fact a 3d solution, just running on 2 axis'. If I knew why it works for me I would share, but I can't see what exactly I am doing differently.
I can tell you my character has a mass of 1.8, so I don't think upping the mass is a solution I would go with since it adds other issues.
Also, I can make my platforms go extremely fast and the character doesn't move an inch. Also my platforms do not have a rigidbody.
The only other thing I can point out is, my platforms move using the "iTween Move To" action. You say you move yours with velocity?