Raise the Z axis about 50 cm and check the horizontal gap between the lead screw and vertical smooth rod by the Z flexible coupler, and by the lead screw nut. If they're not the same you may get some banding issues. If they're not the same for both left and right, you may get some banding issues that'll look fairly odd.

Are you getting any noise from your linear bearings? If they're on their way out they may be exascerbating the problem. Consider replacing with igus bushings.

If you've been upgrading your printer, seriously consider replacing the hobbed bolt. You can at least eliminate this as a source of the problem.