Hi!
Probably you have a nose heavy plane! Move battery and servos back! Do not put on lots of lead!
You said you had a wing cord of 283mm and C of G at 70mm from the leading edge. That is aprox a C of G 25%. Could be a littler too much forward.
Place it at 80 - 90mm instead.That will make the your CG around 30% Which is just fine for all sportplanes.