I'm encountering a similar problem with my (brand new) ultimaker 2
here it is:
http://imgur.com/yiJKYnM
at some point my print is brittle. it seems, that somehow the extruder doesn't put out enough material
I've running the a adjusted flow rate at 115% and a nozzle temperature @ 225C° for PLA.
I'm curious could it the the mentioned retraction setting issue be? @Didier Klein