It looks like it may be your retraction settings. Do your retractions occur in the corners occasionally? For example, go to the Layer tab and tell all retractions to be chosen "closest to a specific location". That will group all the start points together. Then does the issue only occur where those start point/retractions take place?
This seems more indicative of something that would be solved with extruder pressure control (aka extruder advance). You can also solve it by increasing the jerk speed and decreasing the print speed. Coasting can help if it only occurs where you have retractions. Reducing retraction length may also help.
The root cause of this issue is that the extruder continues to ooze plastic after you stop extruding, even when you retract. You cant pull the molten plastic back, only reduce the pressure on it.
@layerone: I think Simplify3D only does start and stops at vertices, so not really. I noticed that when using the "Start at point closest to" option with a coarse sphere. The start/stop points followed the vertices of the sphere, rather than the exact point closest to the specified location.