Also what kind of infill are you using and what %? How tall are the things you are printing? If you monitor the print as it goes, can you see the height of the infill lagging behind the height of the shell? I think that is the idea with changing the speed of the infill - maybe your extruder or hot end isn't able to deliver plastic at the rate required for the speed of your infill, which means a thinner bit of plastic gets laid down for the infill, and over many layers it adds up. Do you notice your extruder motor clicking from skipping steps, or grinding filament that won't extrude fast enough? This is what you get if you try to extrude faster than your hot end can melt it (or if your nozzle is either clogged or too close to the bed and can't extrude at the rate it is trying to). You may even just need to tweak your extrusion rate (or speed) so that more plastic gets laid down in the infill area.