RE: Biplane trimming problem
Is the wing dropping or is it tip stalling? If you turn too sharply the change in air speed from up wind to down wind is 20 mph if the wind is 10 mph. This assumes it takes the aircraft a few seconds to increase its airspeed after turning down wind. If you are flying at a low airspeed that 20 mph drop in airspeed may be enough to stall the aircraft. Another thing that could be happening is torque roll if you are changing the engine rpm a bunch.
Just a couple ideas for you to consider.
Good luck,
Ralph White, Neoga, IL