Extrusion and stepper motor moving inconsistently...

Hello RepRap Community,
I recently completed building a RepRap Prusa Mendel I2 from NWRepRap. All of the axes are working great and everything seems to be in order except for the extruder/hotend. When printing, the extrusion is inconsistent- this is visible in the attached image. I have tried adjusting temperature up and down, changing the settings for filament diameter, and calculating steps per mm for the extruder motor, but the problem remains.
Another problem that I noticed that may or may not be connected to this is the stepper motor. It can extrude at slow speeds just fine, but when retracting or extruding quickly, it buzzes and vibrates. I have adjusted motor current up and down with only slight variations of the problem.
Thanks, icecats
The image can be found here: [drive.google.com]

Edited 1 time(s). Last edit at 11/14/2014 12:19PM by icecats.

Geared or direct drive extruder? Have you done small adjustment to the stepper driver or you have gone all the way up and down? Have you tried to reduce your feed rates for the extruder in your firmware? What size nozzle you have? What width are you using? What extrusion multiplier? What hot end are you using? Material and temperature? For the pictures. You might have some other little problems too, but lest focused on the extrusion first.