I have Great Planes Extra 300S 60, with an OS SurpassII 120 I built from a kit. I am having a bad problem with tip stalling that at times is unrecoverable. This last time it was on a landing approach. Now that I have to rebuild it again, I am looking for ways to help tame the tip stalling. I know it will always tend to tip stall because of the wing design, but I have thought about twisting in some washout with the covering, will this help? The wing is a flat tip design, and I read in an article not too long ago, that this creates more of a vortex at the tip, and in turn gives less lift at the tips. Is this why it tends to tip stall so bad? Would it help if I built rounded tips that will give about 3/4 of an inch to each wing tip instead of the flat tips that it has? Would that help give the air a cleaner exit off the wing?
Please give me your thoughts on this problem.

Thanks for your help.

Should I mention I fly at 7000ft above sea level?