Shoulder instability during band pulls

When I'm working on the supine band pulls, I'm struggling with the hands overhead position. In particular, my right shoulder wants to buckle and collapse in internal rotation. Incidently, I experience the same thing in yoga in warrior 1 position when hold the position for a while. I don't feel as though it's a mobility problem, so much as a stability issue. For example, I can do an overhead barbell squat with decent form. 

I'm interested in hearing suggestions about how I might address this issue. Are there supplementary exercises that might help? My tendency is to try to fight the buckling by externally rotating my shoulder, but I'm not sure if that's appropriate. 

Thanks. 

Hi Nicholas, simply try to hold a bit of external rotation to adjust the stabilization of the upper arm. you should necessarily avoid the internal rotation.