Sometimes the problem is just too much dust on the motors. Wood dust and grease can get pretty sticky over time. This is especially bad with lead screws as the dirt is collected inside the nut. Just clean the motors and guides, put some petrol on them to get rid of old, sticky oil, clean again and relube.
I would look for worn bearings (or perhaps mounting bolts, or even a loose set screw on the motor couplings). You might even go to the extreme, of removing the motor coupling from the drive, and try turning the drive by hand. If you can't, then you have some type of binding issues in the mechanical drive on that axis. Again, look for worn bearings, etc. I once had this issue to appear on my Y axis, and discovered that it had been dragging a shattered bearing around (instead of it smoothly rolling along, as it was designed to do).
This could also be caused be a loose sprocket appearing like the motor is loosing steps.
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Why is my motor loosing steps