I have been printing fine for several weeks using my Geetech, Bowden extrusion, E3 hotend and 0.3mm nozzle. I thought I'd change back to the original 0.4, after changing the settings in Repetier, and in Slic3r, the print was rubbish. It was under-extruding badly, whatever I did. I changed the settings back to 0.3, problem solved. I checked the extruded filament (PLA) and low-and-behold it was 0.31mm average over a 100mm. Looks like the nozzle size is wrong. Has anybody else experienced this?