The coil packs are a funny thing. They have a lot of symptoms. Temperature? is a major factor, mine used to only stumble when it was warmed up, some coil packs might stumble when cold. Some under load, and some at idle, it's a hard to pinpoint issue and sometimes doesn't really make sense, I guess the coils inside the coil pack crack in different fashions? I don't even know. But with these cars the general consensus for stumbling and misfiring is something wrong with the MAF or coil packs and spark plug wires, or both. Usually fuel delivery is not an issue.