I was having underextrusion problems after moves with lots of retractions (even though benchy was working just fine ) so I decided to take the extruder apart and clean it. After reassembling it I was trying the extruder and it worked fine at low feed rates but at high feed rates, 30 and 60 mm/s, it rattles and gives me the phase may be disconnected warning (can't remember exactly the text and don't want to reproduce it in case it damages the driver). The warning might be because the bearing has too much resistance and slows the motor but if I loosen it, it rattles even more.
I checked the crimps with a multimeter and they were all fine.
The calibration is also fine and extrudes the right length at 450 steps/mm.
The extruder's motor has a planetary gearbox , here's the closest model I could find from the same brand: http://www.wantmotor.com/product/42byghpg.html (only difference seems to be the current, mine has 1.3A rating)
The original guide says to connect it to ramps in the order black - green - red -blue with the black wire facing the usb/power connector of the ramps board and I have mine on the duet as on the wiring diagram: red - blue - green - black (as far as I know this only changes the rotation direction).
Thanks for helping