Skipped layers

[Reaping miner]

Update:
I have printed some more of my diagnostic cubes with nut trap. And this problem is very repeatable. I have also taken apart my extruder and X axis, cleaning parts and reassembly. Checked various screws for tightness etc. No change. "skipped layer" problem is still there. I also want to add that i adjusted the flow ratio (92%) to account for the increased diameter from 1.75mm to 1.9mm.

However, i marked and watched the Z-axis motor couplers during the problematic layers. And they indeed turn half of a quarter turn (makes 1/8th) with a pitch of 0.8 mm makes 0.1 mm as the printer is supposed to move. This does mean that the problem i am seeing is not so much as underextrusion as caused by a skipped layer (increased layer volume due to higher height with the same amount of extrusion), but underextrusion for a different reason.

My new hypothesis:
The hydrophilicity of the (PLA) filament caused the filament to increase in diameter. This water vapor gets released in the melt zone and subsequently gets trapped there. (occasionally venting, somehow?) No retraction up to the first seperate layer of the M3 hole in the item. Upon retraction, the trapped (due to increased pressure after some time) vapor flows under the retracted filament. This vapor is trapped under the filament being pushed back in the melt zone and gets extruded before the filament, creating a gap in extrusion -> underextrusion.

This would also explain the large amount of filament that keeps flowing after extrusion stops. The vapor pressure keeps pushing the filament out of the nozzle until it reaches equilibrium with the surrounding pressue. This might also be the reason of the occasional temperature spikes i see at the start of a print (never had that before). It has been quite hot and humid in the printer room the past 6 weeks, usually 28-32 C at >80% RH. And i must admit i have been sloppy with my PLA storage after printing.....

I will try printing with my ABS soon. (It has been opened for a while but i stored it in a closed bucket with a bowl of oven dried silica beads.)

[Reaping miner]

ABS only gave more problems. In addition to the "skipped layer" problem, after a few layers the nozzle started scraping the the print. At some point the infill and wall of the print came loose and i ended up aborting the print. (The infill also looked awful btw....).

Tried some more tivial settings.

-Lower retraction to 0.5 mm @ 40 mm/s (didnt give problems before)
-temp to 235 (ABS)
-checked calibration once again
-lowered speed to 20 mm/s
-travel speed to 100 mm/s

next things:
- check Z stepper voltage (Did slightly reduce some stepper voltages, not sure if i included Z or only lowered X and Y)
- check jerk settings
- General firmware settings check

As always suggestions are welcome.