Top layer not completely solid

Hi
I have a problem with my prints:

full resolution: [i.imgur.com]
As you can see in the image, my top layers are not completely filled, the lines just don't stick together.
However, this only happens in the middle of the piece, where the nozzle is moving at full speed. Everywhere, where the nozzle has to slow down too much material is extruded.
This also happens on the sides. As you can see the edges stick out as too much material is put there, but in the middle of the sides the lines also don't stick together.
It almost seems as the extruder is accelerated at a different rate as the movement.

The problem doesn't seem to be related to the overall speed of the print. I have tried several different speeds now and the problem stays the same.

Has anyone a clue which setting I should tune ?
I use skeinforge (sfact), pronterface and the marlin firmware.

Edited 1 time(s). Last edit at 05/06/2013 05:54AM by tmahring.

Have you always had this, and can it be that it is a firmware steps/mm calibration on your X, Y, Z and E motors?

Found the problem: My hobbed bolt in the extruder had filled up with plastic dust and couldn't get a good grip on the filament any more.
I think I had my retraction speed to high, so every time it retracted it grinded off a little bit of the filament, but thats just a guess.