Hey guys, Thanks for all the info this thread have given me. After a few months I started having serious under extrusion problems on my Ultimaker 2 and I couldn't figure out what to do. Thankfully I came across this thread. I think it should be stickied as it has a lot of useful info (if that wasn't done already )
My problem seems to be too much tension on my material feed when using the the stock Ultimaker 2 feed guide. Once I tried a new feed guide, I had much improved results. Though I must admit that I had to raise my temp to about 235C) once I passed the 5 - 6mm^3/ s mark and some black stuff fell off the extruder; I assuming it was old material that burnt off. As you might see in the pictures.
Attaching my results, now I know what direction to go in, to keep improving my machine. Thanks!