Looks like your extruder calibration and or filliament size is off slightly to me. What you want to do is follow the extruder calibration proceedures in this guide http://reprap.org/wiki/Triffid_Hunter’s_Calibration_Guide and http://richrap.blogspot.com/2012/01/slic3r-is-nicer-part-1-settings-and.html
A summary of that, essentially when you tell the printer to extrude 10mm of filliament, the filliament should move exactly 10mm. To check that, put a mark on your filliament about 20mm up from the topmost flat part of your extruder (accuracy is important, use calipers etc,). now slowly extrude 10mm of plastic and see how far that mark travelled. If you extrude too fast you can get back pressure if you leave the nozzle on that will skew your results, so make sure the extrusion speed is pretty low for that test. Adjust the E-steps value in firmware according to the formula in the guide based on your results and re flash the printer controller, then repeat the test until you are getting accurate and consistent results.
The other thing to do if you haven’t already is upgrade to the new version of Slic3r. The 1.0 variants have a fix for printing round things that will eliminate some of those issues if you have not already upgraded. It does make removal of support material more difficult though.
the other part upgrades (lower X end clamps, 16 tooth gears, etc.) will also help if you haven’t already upgraded them.