Hi!
I've experienced these kind of problem too. Unfortunately, I could print with a certain extruder only. All other extruders are failed. I guess that this specific extruder raised enough pressure on the filament to archive the correct flow. All other extruders, though, transported correct filament when I measured the transportation distance. But on duty, they failed, except this self-printed one. I can't remember which one I used, but I may take a closer look when I'm home again.
But you should take a close look at your extruder to check wether some parts are not as fix and solid, as you assume. Well, the same symptoms does not mean the problem or reason, anyway. But I remember those lines very well.
Kind regards,
cybin