What propeller are you using? Also, you may want to consider that the engine's vibration is getting into the fuel tank, agitating air into the fuel in the tank, and that leans the engine. Once it gets off the ground, the model has more modes of vibration because it no longer has the ground as a partial restraint. You may need to add additional padding around the tank. The airframe may also be a bit light and allowing extra motion, too, so you need to check how solid the nose and engine-mounting area of the airframe happens to be.