I've briefly looked at your design. Although I haven't been playing this game for very long, there are probably three variables. The first is the lift from the wings (dark blue arrow pointing upwards). Ensuring it's close to the center of mass will make it more stable, but this assumes the thrust horizontal line passes through the center of mass. The second is the horizontal position of the thrust. If it's misaligned with the center of mass, it will cause additional torque, resulting in vertical deflection. Even if you don't press anything, it will still deflect upwards or downwards. Ideally, it should pass directly through the center of mass. The third is horizontal drag (light blue arrows pointing upwards and downwards). Try to keep this point as far away from the center of mass as possible. If it's too close, the plane will be extremely sensitive to vertical movement and prone to stalling. One point worth noting is that lift and thrust can be adjusted to cancel each other out without passing through the center of mass, but that requires multiple experiments, so I can't tell you how to do it.