That sounds more like a gimbal issue with the upper arm itself. Certainly an issue I've run into with joints built in the traditional T-pose. In Max at least, leaving the standard rotation order of XYZ leads to gimbal lock by simply rotating the arms down. But switching the order to XZY at least avoids gimbal lock in that…
Your pivots for the clavicle bones are off. They should be centered in the nub where the clavicle meets the sternum, well towards the center of the chest and very far forward. Pivots for the upper arms seem a bit far out as well. They should be slightly below the little bump where the clavicle and Scapula join.…