Actually, tip stall will make it "fall out", not fall out into a tip stall. If it is indeed tip stall ( would not bet on it ) it is common for people to use washout to help. That would be difficult at this point. Piper Aircraft and others use stall strips on the leading edge of their wings to get the same effect. You might try some triangle stock on the leading edge of the wing from the root to about half way out. Maybe two 4" to 6" pieces of 1/4 or 3/8 triangle on each side. This will cause the wing tips to stall last. It is possible to have some "washin" accidentally built in causing a problem. Stick it on with two sided servo tape for a quick check to see if it helps.