Your issue isn't lateral stability. Your issue is tip stalling the wings in a turn. The plane will probably fly ok in a straight line, but as soon as you pull on the elevator to make a turn it's going to sideslip or tip stall (actually quite similar things) and go into a spin. It was hard to see what kind of airfoil you used, but if it's fairly thin and sharp up front that's going to aggravate the issue. Sharp leading edges stall suddenly with little chance of recovery, while blunt LE's ease into a stall more slowly giving the pilot time to react. This could be good experience for you though, since this flight shows you how a heavily loaded plane flies. You'll probably be up around 35 ounces/sq ft in the competition, so learning not to do those hard bank and yank turns will be vital to having successful flights. You need big smooth turns when you're loaded heavily to keep the wing flying.