Only a quart through the motor? You're not broken in yet. But if the plane is light enough, you can fly it in by putting a smaller prop on the plane and running it rich. Try a 9x6 prop on your motor. When your motor is fully broken in, you should be able to set it to a 2 stroke and let it run there without it slowing down from overheating. More than likely you're flying load is just a wee bit on the warm side and pointing the nose up just pushes it over that edge.

Try a smaller prop and let us know.