An uncalibrated extruder can cause overextrusion which looks very similar to what's going on in the picture.
Alternatively, if you're using Slic3r 1.2.9, it has an issue with setting correct extrusion rate in gap fill, which could also cause the problem. Try downgrading to Slic3r 1.1.7 (stable) or 1.2.6 (experimental) and see if that helps.