i have a pizza box flyer with os 40 la. Cg at 5.5 in.
While flying it goes down and i recover it flies okay
Why does it do that.

goes down as in porpoising?

slow it down... PBFs dont fly like a regular plane... fly it nose high and slow or the flexing of the coro will make it jump downward then upward again.

I agree with Unstable. You are probably flying your PBF too fast. Throttle back and keep the nose up higher.......anywhere from about 20 degrees nose up to vertical works fine.

I can remember the first time that happened to me, scares the crap out of you! It's kind of hard to get in your head to fly around in a stalled position, but when you get it figured out you will love this thing.

It sounds like it flies like my mini IFO on the prop not the wing