I agree with Johng. The overall stabilizer/elevator area appears too small for such a short coupled airplane. How does it stall at altitude? when approaching the stall from straight and level flight, can you raise the nose to a landing attitude, or does it stall practically level? The thin section might contribute to the problem. Does it pitch up at the stall, or just drop it's nose. These will all give a clue as to what the problem is.