on the biplane, add some weight to the nose, like a thumbtack or some push pins. They make it fly 100 times better and it won't porpoise on you.
I have 3 Aero Aces and 1 Jet. My labs go crazy when I fly them over and one of mine got mangled when I flew it in the park with them and could not bring it down without one of them pouncing on it. I had it in the air for about 15 minutes and thought I'd fly it to myself and catch it. The dog was going nuts though so between her barking and jumping all around me and controlling it, I fumbled it and she grabbed the plane by the tail and bent it all up.
These things are a blast. I even went to Target on Friday and almost bought another one.
OP, I had the same problem as you with one of mine, but in the manual there is some explanation of how to fix that. I can't remember off hand but it worked for mine and it's worked since.
I fly it in the front yard and in the subdivision intersection and ppl stop all the time and just watch or ask where I got it. From kids to adults, ppl see these things in action and just start to get excited. THey ROCK!