I used to see this badly in Skeinforge, but not in slic3r. I agree with Opus - it's probably where your layers start. Randomizing - as is possible now with the newer versions of slic3r - will mitigate this, but what I see in the photos looks to me as if it would still be visible on a single layer. What are your retraction settings, and do you have retraction being done on layer change. That's the only other thing I can think of that might cause this.
