About a week ago I turned on my 2.5 Rabbit and found it idled very rough. Seemed like a cylinder was misfiring. Stepped gently on the throttle and it went away. Since then I've noticed what feels like a misfire under acceleration, especially when cold, and an occasional "popping" noise when I let off on the throttle. I'm also had a 2+ mpg decrease in fuel economy. Any ideas where to start?