Based on your description I'd say check the bind of the servos. But then I'm sure you do a full checkup on these each time you fly right . Check out Eflites site for the CX or read your manual.

Sorry this is a blanket email I usually send to anyone with CX/2 problems. Some of the other things are valid such as checking flybar, swashplate etc.. as Arrow said. Could also be a battery issue.

Could also be:
Readjust your proportional
In aluminum parts - Lower head is 2 pieces not 1, make sure both pieces are tightly screwed in
Make sure swash is level when you turn on RX and Tx.
I noticed that the plastic from the tail sometime limits the movement of my right servo, make sure it's not binding. A tail boom will also fix this issue.
make sure the pitch connector on the top is not too tight or loose. One blade will pitch up and the other will pitch down causing some funky spin.
make sure flybar is straight ^ will cause same thing
check speed of both motors see if you can see one spinning faster/slower, they should begin spinning @ approximately the same time
Check the movement of the top head, if it lifts at all when you hold the flybar you might tighten this
check center of gravity, could be off pushing it left then bouncing back right etc...
probably not blade balance but could be heat issues
backwash from your blades bouncing off walls
Battery could be low (very dangerous for battery, make sure you don't fully discharge Lipos, I had 1 go dead by just leaving it on overnight in the heli. dead. No worky, $30.
Oh yeah and check the holder on the bottom gear, make sure it's on the flat part of the shaft.

Most people have problems with nose going right not left. Try starting your bird with rudder trim (left, under stick) to hard right when you turn on the motor, then it may initialize this as "center" giving you more nose left trim options. I'd say first check the pot then this.