Mine seems a little sensitive to any resistance. Once I made a spool holder that turned freely, most of the problems have been ones I caused. I've taken to running the throat/heatbreak as close as possible to the extruder itself. And make sure the filament path is as straight as possible. The way it attaches with a single screw can make it easy to misalign. Take a piece of filament and use it as a guide when you install the heatbreak. Move it up and down and make sure it's not dragging on any edges. Don't use a metal tool for that as you can bung up the grooves on your brass hob gear.
My esteps for the extruder are set around 100 for the stock extruder. For testing purposes you could remove the hot end (grub screw on throat) and isolate just the extruder. (You still have to get it hot as marlin won't allow a cold extrude without disabling safeties, but that's just an m-command if needs be.)
Another thing to check is your stepper driver vref voltage for the extruder. I think mine is running around 900-950mv they came set from the factory around 550-600 mv. If you haven't adjusted drivers yet, try bumping that up just shy of 1v and see if it improves. On mine, the dual motor axis needed around 1.1v and the singles a little less.