Looks to me like its the elasticity which is the problem. Reducing the gravity to 10 stops the movement. I have noticed this problem before in some of my own stuff using the revolute joint.#
Rather than being a fixed point as you would expect it seems to stretch.
Not sure of the fix for this though
Just had another look. Try changing your joints to limited revolut joints with an upper angle of 1 and lower angle of 0.
That seems to solve it ( more or less )spongehammer2013-05-30 12:44:38