It is possibly an extrusion width issue. If you are under-extruding slightly, the perimeters will not be quite as thick as Slic3r intended, and bridges and infill can fall short. You can try increasing your extrusion multiplier - you should really do an extrusion width test. I am not convinced that the thin wall test is a good test however, because without the sides of the extrusion being constrained, the perimeter is naturally going to be wider than the infill and cause you to set a lower extrusion factor than is correct. The infill test is perhaps a better one - print a cube with at least 2 solid top layers and 95% infill. When the extrusion factor is correct, you should see very small gaps between the lines of infill, and no gaps between the lines on the solid top layers. If there are no gaps in the infill you are over-extruding, if there are gaps in the solid layers you are under-extruding.
The symptom can also be caused by too much backlash in the printer (e.g. loose belts).
Dave
Edited 1 time(s). Last edit at 06/16/2015 07:42AM by dmould.
