That could be a lot of things, really. I would first check that the pitch is set correctly. I would then look at all the linkages, see if something is loose or too tight, if all three servos have missing teeth, if the swash plate is sliding well on the main shaft, and many other things that would take a long time to explain. Have a look at those sites :

Blade CP repair, lots of good info in there :
[link=http://www.bladecprepair.com/]http://www.bladecprepair.com/[/link]

Ssg Scott's HoneyBee CP2 & Blade CP Mods, Repair, Upgrades and Fixes -Spring 06 thread, very informative stuff in there,
[link=http://www.rcgroups.com/forums/showthread.php?t=494197]http://www.rcgroups.com/forums/showthread.php?t=494197[/link]

Radd's school, in the setup section :
[link=http://www.dream-models.com/eco/flying-index.html]http://www.dream-models.com/eco/flying-index.html[/link]

About Raptors, but a lot of things in there apply to all helis in general
[link=http://www.raptortechnique.com/]http://www.raptortechnique.com/[/link]