i3 calibration issue

Hi!
I've just finished building my first 3d printer. It's a Prusa i3 with e3d bowden hotend. I'm now trying to get the settings correct.
I'm using the hollow pyramid to calibrate the retraction settings. It print quite nicely, but some times the hotend creates a blulb.
This happens randomly. Its like the extruder extrudes to much plastic sometimes.

https://s4.postimg.org/5jcgjurjx/IMG_1847.jpg

Im doing 5mm retraction @ 70mm/s with -.2mm restart distance.
The material is PLA extruded at 190°C. Printing speed is at <40mm/s.

Any ideas why i get this problems?

Hello. I too have a GEEETech prusa i3. mine is the clear acrylic model. There are a couple of things that need to be done to make this a good printer. First and foremost is the holes in the top of the frame that the z axis lead screws pass through. These are a precise hole for the 8mm threaded rod to pass through. the threaded rod is not a precision rod. I promise you it is not straight. you can not take that threaded rod and hold it in an exact position at the top, bottom and middle and expect it to spin freely. This IS your problem. Either cut the rods so they do not protrude into the 8mm holes in the top of the frame, or greatly oversize the holes so the threaded rods do not touch anything up top and can spin out of round freely if theyu want to. Do this and reprint that same part.

Also I have noticed my i3 pro b or c with the mk8 extruder, gt2560 controller with a4988 stepper drivers does not like the .1mm layer resolution. It does well at .2mm