My wife just arrived home and said the engine was slightly rough when driving. I just drove the car and yes it has a slight hesitation but most notably it won't hold constant rpm when stationary in neutral. For example, if I try to hold it at 2000rpm the tachometer needle will move up and down from about 1800 to 2000. It does the same at idle but not as noticeable. No fault light is on the dashboard. My first thought will be spark plugs which are NGK Iridium but they were replaced about 16,000km ago (10,000miles). My next thought is a spark ignitor but there is no fault light. Has anyone experienced the same symptoms and found the solution? (2011 Forester FB25 Manual Gearbox)