Very inconsistent flow

I don't know when this catastrophe started, I just began to realize it when my topmost layer was decorated with gaps. These gaps come from the sags of all of the filament. Near-perimeter infill hasn't sag yet and squished well. (Pic1)

Mine is a Prusa i3 with direct drive extruder and .4 mm nozzle. It wasn't like this before and it happened with same filament, i.e., it's not an issue of inconsistent filament cross-sectional area.
I've tried every setting I could find in slic3r, and cura, but it doesn't change the core problem at all.

To elaborate what happened, I printed a sole rectilinear infill. A layer consisted of many runs. For each runs, the flow is not consistent, ranging from snapping of the filament and thus curling out anywhere it likes, to near-snap thin filament, to perfect extrusion, to saggy, even to a point where when I draw a bridge it just drops to the bed as if I was extruding liquid water! This inconsistency doesn't happen per runs, as in a run the flow can change drastically. See Pic2 and Pic3.

These are the settings I've tried to tweak:
- Print speed 10~100mm/s
- Acceleration 1000~9000mm/s^2
- Top solid layer 3~5 layers
- Extrusion width 0.24~0.4mm & auto
- Cheating with nozzle diameter 0.3~0.4mm
- Changing flowrate 0.9~1.1
- Changing steps/mm 110~160steps/mm
- New autospeed feature of slic3r
- Infill density 10~50%
- Various infill pattern
- Temperature 170~240 deg C
- Layer height 0.12~0.24mm
- Use cura

Edited 1 time(s). Last edit at 06/30/2015 01:21PM by Vashikovich.

What type of filament are you using?

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Master Tinkerer

It's PLA, both 3mm and 1.75mm.
I've measured them multiple times and they're quite consistent.

I am suspecting the nozzle to be clogged since my home was full of ceramic dust recently. I've tried to unclog it by unplugging filament from the extruder at ~100 deg many, many times but doesn't seem there is any difference.

A cold pull is a great idea, just be sure to cut off the tip of the contaminated filament afterwards. If your worried about dust on your filament, try printing one of these: http://www.thingiverse.com/thing:492067 would you mind uploading a picture of your extruder setup?

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Master Tinkerer

When I said I did cold pulls, it all actually failed amd always snapped. I finally made a success in doing that and able to extract much dust. But no effect...

The extrusion feels like it's too liquefied. It always sag, but when I decrease the flowrate gradually, it snaps before it has a chance to go upright.
The extrusion is also like unresponsive. Everytime it stops extruding for a moment, e.g., when changing layer, with out without retraction, it always underextruding for a moment before extruding normally.

This is my extruder setup [lh5.googleusercontent.com]