MINI not extruding enough filament  

MINI not extruding enough filament

This started months ago and I've been trying to figure out a solution with nothing fixing the problem for more than a short time.

The attached image is an example of what I'm seeing. That should have been several solid base layers but as you can see it's maybe half there. This problem has come and gone over the past months, sometimes worse, sometimes better. Various things I've done seem to have helped short term but relatively quickly the problem comes back. Most of the time the first layer looks like it's printing fine, adhering to the print plate and fully extruding into a seamless layer. Then the upper 100% infill layers end up looking like they have holes in them. In the picture attached you can see that the perimeter appears to be fine as the shape was correct, but all of the layers, even the first one, were inconsistent.

Things I've tried:

• Double, triple, etc. checked my print settings to make sure I'm choosing the right printer, filament, nozzle size, etc.
• Different nozzle
• Smaller nozzle (0.25mm)
• Larger nozzle (the picture was a print with a 0.6mm nozzle)
• Different filament (I almost always use Prusament on my MINI now as other brands have been problematic)
• Increasing nozzle and bed temperature incrementally (all recent prints have been PLA with the default temperature of 215, have tried 220, 225, 230, 235. At 230/235 the problem seemed to go away but the picture was at 235 and it's still not extruding enough)
• Raising room temperature and keeping it somewhat steady (space heater and curtain across the doorway. temperature sensor in the room confirms it's been between 30-35 since I set this up)
• Both loosening and tightening the filament tensioner on the extruder
• Disassembling the extruder, including checking the set screws on the extruder gear (were not loose in the slightest) and cleaning the gears and other parts
• Cleaning the nozzle
• Multiple cold pulls
• Disassembling the X-Axis carriage and hotend, making sure there are no obstructions

I do sometimes hear what seems to be the extruder gear slipping on the filament but I can't for the life of me figure out why this is happening.