Two things occur to me. One's the radio, the other is the wing mounting.

If your radio isn't given to twitching the elevator, then I would suggest you secure the back of the wing so it doesn't lift off in flight. Perhaps a screw, or tape, or additional rubber bands.

We see a similar phenomena on high wing planes with rubber band mounts. With only four rubber bands, the front of the wing will lift, and the plane will climb abruptly (usually during a maneuver.)

In a similar fashion, the lift from your wing could be causing it to lift at the trailing edge, causing the plane to dive.

Good luck,
Dave Olson