If the plane is unstable it sounds to me that it is to tail heavy. Hard to say without seeing it fly. But if you have your battery where GP says to put it I am betting its to tail heavy. That battery you have is to light since it is a smaller size than recommended. If you had a battery that matched what the specs call for I think it would move the CG and it would be correct since the bigger battery would be heavier. This is just a thought but I would look at it.

The thing I noticed about these small electrics is that CG can be changed so easily without realizing. I had my CG bounce around by me just changing props sizes. There was a small difference in the weight between the props I tried. It took me forever to realize that my CG was off due to prop size. Each one weight different and was very noticeable on the plane in flight. I did not even stop to think of checking CG since I had done so already when plane was complete. After rechecking I was shocked to see how far off it was due to a prop swap. So I would just keep in mind every change to check the CG each time. Theres aren't like the big gasers which are more forgiving in that area.