In the advanced tab set the width explicitly rather than letting slicer calculate it.
You want to set it to between 1.5x the layer height and 1.7x the layer height, I use 0.5mm for 0.3mm layers.
I'm not sure what gear you are printing, but one of the files I downloaded from thingiverse had instructions to use 0 extra shells in addition to the 90-100% infill. This created more room on the interior that the slicer (in my case skeinforge) could not overlook. My fill came out perfect.