Warts on print

Hi all -

Mendel90 w/Sanguinololu & sprinter firmware (3mm PLA filament, 0.5mm extruder and heated bed)
Repetier & Slic3r

Since getting the bed aligned with the Mendel90 and parameters set pretty well, my prints have taken on a new level of precision - yay!
I still do get periodic 'warts', like this print of Lincoln's likeness. Any thoughts on how to address this?

Thanks in advance!
Rob

The location of those make me think it's the hot end leaking a little bit as it makes its move on the Z-axis.

Could be moisture or other contamination in the filament.

Definitely it sounds like moisture in the filament. Are you listening little explosions when printing? That´s moisture. Try putting the filament for a few hours in the oven at 70-80 C degrees. I live in high humidity region so I am very used to do so, and the prints are a lot better when no moisture is inside the ABS structure