[Mark McLeod]

I haven't disassembled one of the kit joysticks myself, so I don't know what mechanical problems dust and moisture could cause. I suppose long-term storing a joystick all bent over, like it might be if it's thrown into a crate with other junk, might cause a mechanical problem.
If you already had another joystick go bad, then you've got spares to disassemble and investigate.

If the centered point is exactly -.5,.5 that's terribly suspicious. I wouldn't think a mechanical problem would be so exact. I'd lean towards an electrical issue being the cause.

I'd look at the joystick output on the Windows Game Controllers utility to be sure of what is being received from the joystick.