There are several things that can cause this:
1) This looks a lot like what you see when the filament on the spool gets tangled for maybe 20 seconds. The extruder has to pull harder and harder until suddenly the spool turns and the tangle comes free. Make sure the filament isn't stuck at the start and middle of your print.
2) Measure the length of your extruder spring when the extruder is closed and there is filament in it. How long is the compressed spring? It should be 11 to 11.5 mm.
3) Make sure there is no dust on your filament. This can cause partial clogs.
Could you send a photo of your extruder? Also remove the filament and photograph the bolt. Check the bolt grooves to see if they are filled with filament. Make sure it looks like a good grip. Also examine the small black disk that squeezes the filament. Does it spin freely? With only a few inches of filament in the bowden tube, hold the filament back with one hand and turn the gear with the other. The feeder should be able to pull about 22 pounds.
It looks a lot like my problem, I solved it, you can read all about it http://umforum.ultimaker.com/index.php?/topic/2722-printing-big-thing-problem/
Another thing I did was install http://www.thingiverse.com/thing:133353, since then no more clogging, no more bad layers.
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