If it's always faulting at a certain spot 12" from the end of travel in X, it does sound like an issue with those linear slides. If they're misaligned, that can cause this. Try loosening the bolts that hold it down and running it and see if the fault goes away. And yes, lubrication can help. Check for rust spots as well.

But if the fault is consistently following the motor, and not the physical axis, it points to an issue somewhere up the line, especially if changing the motor didn't help. It could be that you've got a bad servo drive, or even a bad cable.