Has it done it since you got it? Or what happened just before it started doing it?

A couple of simple things to check would be see if the mainshaft is bent. Check if your blades are tracking properly. Check the balance of the blades, and the whole head. Also check your flybar to see that it is balanced and the weights are the same distance out on each side, and that the flybar is the same length on each side (or balance that too by weight). Also check the flybar paddles and make sure they are level with the head like it shows in the manual.

What have you tried, and what haven't you tried? Hopefully some of that helps =)
If you need instruction on how to do some of it check out www.bladecprepair.com for some tips, or ask here =)

Good luck with it!