My Ender3 has been having intermittent under extrusion issues. Layers of good density alternate with weak mesh. When printing multiple objects at once, sometimes one will be almost entirely good, while the piece next to it is all meshy; so it's not even consistent within layers, or between sections of the print bed.

I've cleaned it, and replaced everything from the stepper handle to hot end.

Today I've been trying to calibrate esteps, and they're all over the place.
Initially, tests produced from 19 to 23mm (should be 100). I calibrated to an average of those, and then tests ranged 105 to 115mm.
I tried a different spool of filament; that one produced 58 and 67mm.

What can be causing these inconsistent results?