I guess we need to start with the basics. Have you seen and gone through this guide? [link=http://www.heli-wiki.com/mediawiki/index.php/HOWTO_Wiki_Overall_CX_and_CX2_Setup]Setup Guide[/link]

What you are experiencing my be what is known as Toilet Bowl Effect. It's a circular oscillation that just keeps getting bigger and bigger until you either land or crash. And it is very fixable at no cost. It's all in the setup.
If you haven't gone through this guide, I suggest you do. It will get your helicopter rock solid. And that's a very good place to start.

Soloboss