First things first... How long ago was the problem before? Did someone repair something that fixed it then?
If it has been an on going concern does it do it only when it gets hot or does it do it all the time? Trying swapping sides on the coil. If you look at you coil pack, it has 3 separate coils on it, it is not one big coil. Wherever number 4 wire plugs onto the coil, swap it and whichever wire is right beside of it, meaning side to side of the vehicle, not front to back.
Had this problem while i was working at a local dealer. Came down to it being the cylinder head was cracked between the valves and when the engine got warm, the crack would let coolant leak into the chamber thus causing a misfire. When the vehicle was cool, either the crack was closing up or the lack or pressure on the coolant system would keep coolant from leaking. Try doing a "hot" coolant pressure test. Put the pressure tester on the radiator and get the vehicle warm with it installed and try to add pressure to the system. If it goes over 16 and stays or goes higher, you should be fine. If it wont build up pressure no matter how long you run it, I would be looking towards a coolant leak in the cylinder.
