What causes this kind of layer issue?  

What causes this kind of layer issue?

I've been having a devil of a time trying to print anything at all on my mk3. But sometimes I get prints that finish and the extruder doesn't just stop after 30 minutes with a 'click click click.' It's rare. But it happens. When it DOES, though, I get really odd lines in my layers. What can I look at to remedy this (which might help me figure out what the issue is with my extruder)?

The first layer always looks fine. That's easy enough to get adhering.

The problems come later.

Things I've done to try and narrow down the cause:

1) Adjust idler tension (over and over and over).
2) Replace thermistor
3) Replace cartridge heater
4) Replace heat block
5) Replace heat break
6) Replace PTFE tubing'
7) Replace nozzle (multiple times)
7) Upgrade to the R3 extruder design
8) Replace extruder motor with a much higher torque stepper motor (which stays cool and doesn't have the super hot issues the stock motor had)
9) Tried a dozen plus different kinds of filament from the ultra expensive to the cheap (cheap actually tends to work better).
10) Adjust heat settings
11) Different nozzle diameters (0.3, 0.4, 0.5, 0.6) -- all the same issues.
12) Tried printing at much slower speeds.
13) Checked tension in the X and Y belts as best I can.
14) Tried different built plates.
15) Recalibrated. A lot.
16) Probably some other things I can't remember.

None of this makes any different in my success/failure ratio (which is about 1/10 at the most -- possibly 1/15), with the only actual problem being that eventually, in most prints, the extruder starts clicking and stops extruding. I can usually push the filament down past the hang and it will pick up again, but tightening the idler pully only helps sometimes and loosening only helps sometimes. Eventually, it just stops extruding and a print will fail.

When it DOES print, I get these odd lines in the print that make it look like something just isn't... right. But I can't figure out what.

What can I look at?