Heli is correct about the flat bottom airfoil not wanting to fly inverted very well. But I think if you adjust the CG enough to get the elevator trim neutral at level flight you will have much less down elevator pressure when inverted. If you lighten the nose a bit the tail will be more effective throughout the flight envelope. Don't go to far back, make adjustment small and test fly until it is better, then quit. The old saying goes: A nose heavy plane will fly poorly, a tail heavy plane will fly ONCE.