Try changing your layer height slightly. Some people claim that there's an interaction between layer height and the stepping of the Z-axis worm gear where the layer height isn't an integer number of steps, causing the Z axis to be too high/low occasionally. For example, this discussion:
http://forums.reprap.org/read.php?340,313617,314368 .
Given that you're seeing regular bulges, and this sort of rounding error would occur regularly, that'd be my guess.