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 <img src="smileys/smiley24.gif" border="0" align="middle" />
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 )