Equally spaced surface flaws
The surface defects you describe is caused by Z-wobble. Basically this leads back in a defect in the Z axis gantry where X and/or Y movements are transferred into the print head. This can be cause by:


a damaged nut, 
lead screw not straight, 
stick-slip, 
binding, 
lead screw out of alignment with the coupler, 
lead screw touching the stepper shaft in the coupler, 
the Z guide (at the other side of the X-axis arm; the one without the extruder mounted on it) has too much friction of the wheels on the profile,
play on the carriage, 
lead screw should be the same distance from the extrusion profile from top to bottom, 
etc.