Jim,
I have the 60 size GP Extra with a OS 91FS as well and the 33%. In my opinion, the most common cause of poor flying characteristics is excess weight. You have a large engine, and possibly some firewall reinforcement, a big fuel tank, and oversize battery, etc. We live in a world of bigger is better. I'm not discouraging big motors, but you will pay a price in handling. Experienced pilots usually don't have trouble, but it could be part of your problem.

This plane is capable of flying nicely as shown on the plans. I wouldn't try warping the wing unless you are seriously interested in experimentation. Try landing a little faster, that should eliminate the tip stall. Landing in grass will help you slow it down but if you do this, get some larger wheels to avoid snapping off the gear.

Tom