if you know how to use a multimeter you should check each of the coil pack plugs for a 12v feed.

if this is going abit far remove the spark plug cover start the car and wiggle the wires that go to the coil packs, try bending the wire in different places and see if it improves or makes running worse, if it does either it needs replacing.
you can also cut away the black insulation on the wires near to the plug, the wires inside will most likely be brittle and the inner casing cracked.