Oh did you tried smaller layer height ??? It make the same to me when I print to smaller layer height and rectilinear infill…
I haven’t tried changing layer height @Voxel_Factory , been printing at .2 for a long time. Do you recommend going bigger or smaller?
Mmm I see, on my side I start seeing this at .12mm, no problems like this at .2 for sure. I’ll keep foloowing this post and throw ideas if I have any. Maybe try 100% infil with your regular settings and see if there is kind of dotted patern wich would confirm there is under extrusion for an unknown reason.
Still wrestling with this one, now it’s appearing in the shells as well (I’ll post an updated photo once I capture a good example).
Since my last update (after cleaning & overhauling the nozzle) I’ve replaced the main extruder block (was worn by filament), cleaned & examined the drive gear (stainless steel, driven by geared stepper), examined the idler bearing for wear and drag, replaced the extruder stepper driver, switched extruder “channel” (using E1 spot on RAMPS board instead of E0), updated to the latest Marlin firmware & re-calibrated (as best I can with it printing this way), tried different/bigger power supplies, multiple brands/batches/types of filament (PLA & some nylon), different controller software (printrun, octoprint), different computers, different physical locations and different slicers but the results is always the same, or as I said, getting slightly worse.
I’ve also tried many variations of slicing settings, increasing extrusion, reducing/removing retraction, different nozzle size specification (thinking maybe it’s worn and larger than spec.), slowing way down (15mm/s) speeding way up (100mm/s) and anything else I could think to twiddle with 
I’m not sure what to try next. One thing I’ll add, the first few layers of the bottom of a piece turn out very nice, but as soon as it starts building up, this pattern emerges. Printing 100% infill is better, but still exhibits the problem.
Which hot end do you have btw? I recently got this kind of half-jams with an E3D v6 and CFPLA (carbon-fiber stuff from protopasta). I fixed it by going faster! The slower the more it was impeding the flow (probably b/c the filament conduct heats upwards and make it stick to the barrel). Did you try to change your hot end barrel? It may be dirty beyond repair?
J-Head, cleaned very carefully but it could be worn (few kilos of PLA have been through it).
What’s weird is how the first 2-3 layers go almost perfect before this starts, and I only run the first layer slower than normal (50% to make it stick).
I’ll try going faster and see it changes though, can’t hurt
may be the heat is slowly moving up until half-molten filament starts to stick to a notch/scratch in the barrel… :s