Usually there will be an optimum aoa. Apart from the lift and drag forces, there is also a aerodynamic pitching moment which tends to rotate the airfoil nose-up. So an airfoil by itself is typically unstable in flight. Now to get a larger glide range, you can try varying the angle you fix the airfoil to the glider. Keep it under 10 degrees though.
Lift tends to drop off rapidly after that.
Assuming your aerofoil is rightly shaped and generates lift, I suspect in your case the lines of action of the lift force and the weight of the entire plane do not coincide. This will result in a pitching moment which will cause the nose of the plane to pitch either upwards or downwards, resulting in the tumbling motion observed.