Is it with application of power? It sounds to me like the wing or stabilizer incidence is wrong or the engine thrust line is wrong. I really doubt it's a radio hit if it's the same thing happening over and over.

Also, if it's not balanced properly then it will never be in trim for all speeds. Most planes are set up to be nose heavy because it's "safer." The fact is a nose heavy plane won't fly right, it's just not as sensitive as a tail heavy plane.