I have Mendel with Adrian's geared extruder. Printing quality has always been uneven. I noticed that filament driver slips in spite of tight springs. Obviously driver teeths are not sharp enough.
I tried to make a new driver from bicycle inner tube valve cap like this.
Lathed first two M4 nut to fit inside the cap with power drill.
Clued all pieces together.
The final driver assembly
Much better printing quality