Not all the up on MadCatz's variations... is this the stick that looks like the "Standard Edition"? Could be one of two things... either the button is going bad, or the PCB is starting to go faulty.
If it is like the SE stick, then it is fairly easy to take apart (6 screws on the bottom), remove plate, and then you have access to all the buttons. You could try rewiring "X" to another button and just use that as a test... but replacing the buttons is just a matter of ordering the parts, disconnecting/removing old button, replace/reconnect new button and off you go.
If it is some other design (like some of the low end Hori sticks), then it may required desoldering/soldering, etc, assuming the PCB is still working properly.