I noticed (and later saw mentioned in troubleshooting tips) that the Z axis motor on my machine can move up and down a little bit. I replaced two of the washers with one M3 nylon washer, 1mm thick. This helps, but it is not perfect. The tricky thing is that the motor should be free to wiggle sideways, and screws, I think, should be screwed tight (bottoming out) so they don’t come loose. So the spacing is delicate.
Nylon is a good choice, good friction.