To really diagnose the problem we need to know the airframe and power plant combination, total dry weight would help as well.

Your cg should be set to where the nose drops slightly during an inverted 45* upline. (Point nose 45* up, add power to not lose airspeed, Roll inverted, let go of elevator; If the nose drops to the horizon - you are nose heavy; If the plane climbs - you are tail heavy.) Move stuff accordingly and repeat.

You should have to hold slight pressure on the elevator to maintain level inverted flight.