Bowing indicates the servo is moving beyond what the pushrods can sustain. You should set your TX endpoints to prevent this from occuring.

Once you have the endpoints set make sure that the elevator is held in place when you apply a light force upon it. If it moves easily, air movement may create a harmonic rise and drop ( much like VERY SLOW flutter ) causing the plane to waffle up and down.

You said your plane was "porposing", by that I take it you mean it was moving up and down.

Bare in mind that many different things could be in play.

The plane's pushrods may not be set up properly.
The elevator may not be holding position.
Downthrust combined with initial trims not being right, can cause the plane to pull down as you throttle up ( or up ) and up as you throttle off.
etc. etc.

A true C.G. issue is determined by flying the plane upright, then again inverted.

If the plane climbs both upright and especially inverted the C.G. is too far back.
If the plane maintains level both upright and inverted, the C.G. may be OK, but too far back for a Warbird ( great for 3D though! )
If the plane enters a slowly increasing dive from upright and a bit faster when inverted, the C.G. is fine.