I've been struggling with a strange localised underextrusion problem that after a kg of plastic I've nailed a fix. I have many beautifully formed and accurate test pieces laying around, but it didn't fix the issue. A combination of inceremental changes to extrusion speed, retract distance and extra restart has fixed it. I got around to doing that last, if the guide had been out a few weeks ago it may have helped get to the solution faster.