It might be just oxidation, or some bit of dust has worked its way into the mechanism. I don't have that particular joystick, so I don't know if this will work or not, but when it happens to me with the Logitech Extreme 3d Pro, I find it helps a lot just to move the joystick all around for 15 minutes or so, twisting all of the axes several hundred times. That seems to clean off the oxides, and gives me several smooth months of use thereafter. I hope this helps!
-Peabody-