When it is set to random you see a gap on every layer where it start/stop the external perimeter.

I am quite certain it is in the slicer program. I used Cura in the beginning because with the same setting it was faster but Slic3r was always more accurate on dimensions and cleaner. I have check and double checked all my settings. I guess the only thing I don't know for sure is the nozzle diameter, I read that it was .4mm but I have never measured it. I might try KISSlicer but I love having the slicer in Repetier Host. It is just so simple to make a small change and click print.