Undulations in the walls of a print may be due to wobble in the Z axis. A thorough analysis of the potential causes is given by whosawhatsis1 in his article "Taxonomy of Z axis artifacts in extrusion-based 3d printing"2, however one point of particular interest for users of Slic3r is the wobble caused by motor steps not matching the pitch of the Z rods thread. This can be addressed by ensuring the
Layer Height setting is a multiple of the full step length.
The relevant part of the above paper is quoted here: