I've been experiencing the same exact problem for a few weeks now and still don't have a clue what to do.
Eventough my teflon coupler has been replaced 2 months ago, I put in a new one, but not even that helped. Aside from that I did everything you did as well. Tightened the belts, oiled the rods and the leadscrew, cleaned the nozzle and the feeder, calibrated everything just to be sure. The only difference I can see is that I'm using the stock feeder. Aside from that our cases are identical.
I'm experiencing this with all my Colorfabb filament. Most of the spools are at 70-80%, so I'm not even close to the inner coils (not that it'd matter at printing speeds of 40mm/s).
The weird thing is that all of these spools (Dutch Orange, White, Ultramarine Blue and the Blue Gray) printed perfectly when I unpacked them.
I'm printing with the rest of my Faberdashery filament now, which seems to reduce the problem significantly. I don't need those higher temps or increased flow anymore. The problem isn't completely gone, but basically reduced to 2-3 slightly underextruded lines per surface. The walls are much cleaner and more even as well.
Have you tried any other brand besides Colorfabb? Just to get a comparison. Are your walls kind of "wobbly" and uneven as well?
I just don't get how this could happen more or less over night. I store my filament in a dark, dry room and some of the spools are only 1 month old. Plus, like I said, they printed fine in the beginning. The diamenter is unchanged, 2.84-2.86mm.
Going back to Faberdashery is an option I'd like to avoid as I still hate that loose filament, even after going almost a kilometer. That and the fact that I have 12 spools of Colorfabb filament... which I'm now too afraid to use.
The one thing I didn't check or exchange is the bowden tube. If it's a frichtion problem that might explain why the Faberdashery filament is less problematic (as it's much smoother and harder).