The way I see it, not sure if correct:
When coilpacks get old, they tend to crack/split and with that the current isn't concealed and you get an uneven current to the spark and therefore missing.
Normally the cracks are hairline and you can't really see them, but sometimes you do.
Best bet and to save you money. Borrow another set if someone is kind enough down that way.
See if its 100% the coilpacks. If it is, replace them, or if you can be bothered, work out which one it is and replace.