[pfreivald]

Your arm seems pretty rickety, especially for the weight it is trying to support. Forgive the paint-CAD, but you can add a lot of rigidity to a thin arm with some supports and steel cable, while adding almost negligible weight.

(Also, I agree with the neutral balance suggestions. We're using a four-start screw and a FP motor on a 64:1 BB for our lift, and we can stop it anywhere we want and it just stays there... It helps that we can lift the arm from behind the pivot with one finger!)