Constant clogging  

Constant clogging

prebuilt Core One, a few weeks old, 5-6 days total print time. 

After a rough period of initial issues, it's now unuseable. 

0.4mmHF nozzle that came with the printer. It clogged a few times in the beginning. I blamed the filament, so bought a dryer, built a drybox, and discarded a lot of prints. Occasionally it printed acceptable prints, but sometimes clogged for no reason I could see. 
The last few days I can't print more than a few minutes before it invariably clogs up. 

So, idler and extruder gear looks fine. Clean, no debris.
Nozzle looks fine to me. I've had it out, the filament goes through about to the threaded bit. Needle goes through easily after heating and prodding it for a bit.
Looking at print history, the temps seem fine, it's holding steady at the preset temp, and fan speeds look normal, as far as I can tell.
Using Prusament and other filaments, no discernible difference.

At this point, my new printer is broken. Any tips to how to fix it, besides changing the nozzle and hoping it doesn't happen again?