Slower landings
I would suggest that you check the following:
1) There is washout in the wingtips (mentioned above)
2) The washout is the same for both wingtips (mentioned above)
3) The airfoil is actually the same at each wingtip (contours accurate)
4) The wingtip leading edges are not sharp (blunt is better)
5) Verify aileron movement in 'down' direction does not 'induce' a tip stall
6) Wing is balanced correctly (apparently it is)
I don't think there is much else that influences a tip stall. Washout seems to help sooooooooo much that I would really make sure washout is present and the same for both wing tips. I have played with minimum to excessive washout and it is suprising how much it helps in control during a slow, nose high landing.