Well, to begin with, we need a little more information. Is this a Precision Plasma kit? If so, which one? The gantry should travel smoothly, unless there is something flexing or binding. Is there a gear/belt reduction? Is it rack and pinion? What is the pitch of the screw on the z? What C&CNC kit did you get? the 300oz, or 620 oz motors? What is your velocity/acceleration tuning?
Lets work through this one thing at a time, and you'll be up and running in no time.