I have heard of several ways to fix this issue. My new, and first electric 3D plane, Efilte Ultimate climbed inverted. I added just a bit of flaps and it corrected it. I had read that worked for someone in an RC mag a while back so I tried it. Quick, easy and free.

The other thing several folks with my plane have done is mount the battery higher up, course mine is biplane. My battery is mounted as high as possible. So the flaps was my next attempt.