How heavy is your plane? I have had 2 sea masters and really love the way they fly. I do not think washout will help a lot if the plane is in the right weight and balance range. It has pretty good flight characteristics right out of the box. I am interested in the foam core wing though. Who cut it? Are you using dual servos for the ailerons? Also what are your rates and how fast are your servos moving?

Going from a 60!QUOT! wing to a larger wing should make for a better wing loading unless the added weight overtook the added span. Do not know quite the reason you have a squirrelly bird. Mine, and everyone I know that has them, has a solid flying plane.