I've been fighting this hot end for over a week and I continue to get major under-extrusion. 1. Installed official E3D hot end 2. Printed 1/3 of a marvin perfectly, then underextrusion began. (Had nice marvins with old hot end) 3. Next print was 100% underextruded. 4. Updated firmware and told the printer which type of thermistor I have and did a PID autotune. 5. Underextrusion continues about 1/2 way up the marvin. 6. Swapped out the bowden tube and top bowden tube connector on my printer for more secure fit. 7. Tried 3 different types of filament. Underextrusion continues. 8. Found the heat break was not fully seated in the cooling tower. Took apart, checked over, reassembled with the heat break more secured into the cooling tower. Heated and rechecked for tightness. Got an EXCELLENT print. Next print (without changing anything) was back to underextrusion about 1/2 way up. 9. Found that the filament can be difficult to push through and manually extrude. 10. Dissasembled hot end, changed out nozzle with brand new. Double checked for blockage or burrs. Checked for nozzle to have a gap between the heat block and the nut area of the nozzle. Tightened and heated and checked. Problem remains. 11. Put the old bowden tube back in (one that has the curves needed, as in it's been in that shape long enough that it's not being forced or pinched in curves). Printed and got higher before underextrusion happened. Canceled print and tried to push filament through by hand, was VERY difficult. Pulled bowden tube and installed short bowden (the one that comes with the hot end) and pushed the filament through, it was difficult at first, like it was not lined up, or it was jammed, but then all of a sudden it went swoosh, and the filament went through like butter. Is it possible that as the hot end advances up, and the bowden tube bends more, that the tube inside the cooling tower is being forced slightly off canter and causing drag? Anyone have any more ideas? I'm about to swap back to my old head and forget this headache. I've spent soooo much time and filament with this problem.