I don't think you can truly counterbalance a built up balsa aileron by hanging a weight off the end. I think the problem is a run-away flexing condition. A servo could be added to the outboard end of the ailerons to see if the problem goes away. I'd be willing to bet that a pair of standard servos per flap would do the job, it isn't a matter of torque or holding power, it's about controlling vibration by adding another control point.