>>8112910People will probably call me a retard for it, but I used a blow dryer to warm up Jiren's arms from the bicep swivel and pull them out and push them back in several times until it became smooth. Kept the arm straight, no twisting and turning, no excessive force.
It works like a treat now, though my Jiren's right arm (where it enters the chest) is still creaky, but that's unrelated.
