Stall strips
I noticed a guy at the field this weekend with stall strips (about 10" of triangle stock balsa) mounted to the LE of his 1.20 size extra/edge/cap thing near the wing root. Looked just like the crop dusters I worked on when I was a kid. He said it solved a horrible tip stall problem. If you only notice it when pulling hard you may just have a split elevator problem. If you can, dial some more up in that left elevator and see if that works.