Mark, post your xml file and I'll look at it.
It's entirely possible that some of the hardware has given up. Can you swap your wires going form x or y (don't try this with an axis that has 2 motors) motor to z motor easily? You could start narrowing it down this way. You have 2 known good axis and one dude. By swapping wires you can narrow it down. Really only a few things it can be as far as hardware goes. Motor, drive, power supply for that axis, breakout board, or wires between any of them.
Brett