Quote Originally Posted by irving2008 View Post
then you probably have either a bent screw or a dodgy ballnut...

If your X and Y screws are the same pitch then swap the x and y nuts (being careful not to lose any balls). If the problem moves, its the nut, if it stays put, replace the screw...
Ermm I wouldn't do that really, it's not a conclusive test for a worn nut and any wear could easily be masked due to differences in the screws.
If the nuts worn enough to cause that much wazy-ness then it will be quite obvious and easy to feel or see without going to that much trouble. A bent ball-screw would show more has a vibration and wouldn't really show has a consistent uniform wave.

If It's mechanical issue it's more probably going to be a floating ball-screw due to lose or worn fixed end bearings or some other lose assembly or component.

Like I say some pics and possible file your having problem with would help.