When you say it noses down when inverted are you trying to say that the nose drops towards the ground or it is pushing towards the belly?

From what you're saying above if it's pushing to the belly when inverted I'd say it's tail heavy.

The way to test is trim for level flight at about 1/4 throttle then push over to a vertical dive and let go of the stick. If it pulls up you're nose heavy and have too much up trim to compensate. If it trys to tuck to the belly then that confirms that you're tail heavy and are running with down trim to compensate. For an aerobatic model of this sort it should run a true vertical line with little or no nose up tendency but definetly no tucking.