here are a few possible areas that it could be.
*coil packs.
*coil pack igniter.(if any of the wires leading from the "coil pack igniter" are not giving a reading/or any that are reading spastic on the multimeter.you have a problem.)vise versa for the wires that lead to it from the computer.
*Crank Angle Sensor.(if the computer gets the signal to fire the coil packs at the wrong time you definantly will have a missfire.)
*exposed wires in the ignition system.(that means a thourough check of all the wiring in the ignition system.)
*spark plugs.
*water in the airbox.
*leak in the intercooler to manifold pipework.(the car will spit out black smoke,right after it splutters,if this is the case.)
*afm is a possibility.though dont quote me on that.its only a suggestion.
*the computer.