Short answer is that it noses over on launching because it's going too slow. Be careful about trying to gain altitude immediately, because putting the nose up just slows the plane down. A straight fly out for the first 2-3 seconds is much safer. Without actually seeing your plane fly it's impossible to say, but it may be nose heavy as well. If it tends to drop its nose and glide fast when you cut the throttle and if it's hard to flare for landings a slight shift back in the CG is in order. That will improve both flying and launching since it will make the plane less prone to dropping its nose when going slow.