Coil pack then? Try and find the one which is missing buy pulling plug leads, then when you have, swap an HT lead with another one to make sure it's not that. If you've done that, swap a plug over to make sure it's also not that, Once you've confirmed that it's not an HT lead or a plug then you'll have proved that the coil pack is dying.
Oh make sure you don't get a couple of plug leads mixed up in the process, do a drawing of the coil pack on the engine and mark which plug goes to which cylinder so ever if you swap everything you don't end up with problems.