you might try checking the motors to see if one is failing, or you might check and make sure that both sets of blades spin freely. If there was some binding it would cause that problem

also you can check out the cx2 guide and see if you can find anything there. It's a pretty simular heli and it is a good resource.
http://www.heli-max.net/mediawiki/index.php/Index:HOWTO