My wagon has this problem when I first drive it for the day. It idles a little rough, but doesn't usually stall. When I start the drive, the engine will just start to sputter and hesitate, and then backfire. Usually after doing this a few times, it will go and it won't happen again. What should I be looking at to fix this? I guess I figure it is an air or fuel flow issue, what would cause this when cold but not once it is warmed up? Thanks.