IS this missfire evident at idle, or when driving or both? MFI systems are very sensitive to air leaks, more so at idle for obvious reasons. It would have to be some pretty serious overfuelling for it to cause a missfire.

As for the comments about the sensors, the auxiliary air device is merely a means of controlling the engines idle when cold, if it was faulty you would see abnormally high idle speeds when hot. The coolant temp sensor would almost certainly make it a nightmare to start from cold.

As someone else said, a compression test is a good place to go now. And remember also, air leaks can be ANYWHERE on the engine itself (as its connected into the breather system), and that they are not just limited to the intake pipes. The brake servo leaking air is another favourite that is overlooked.