Last month's puzzler happened before our new forum was set up. We accepted lots of replies via email but nobody solved the mystery!
As a recall, we asked why the top solid layer was making waves and not laying down flat. If we zoom in on the picture you may notice the last layer of infill is oriented parallel to the solid top layer. Some "roads" or "tracks" of the solid top layer have nothing underneath to bond to and lifted up as it cooled and contracted thus forming the waves.
To avoid this you will need to verify the last layer of infill is printed orthogonal to the solid top layer. In Simplify3D use the toolpath verification and visualization to ensure the correct orientation of the infill. If required, you can change the infill orientation on the "Infill" tab in Simplify3D.