You say it runs rough at start up, does it clear up after? Where did the replacement parts come from? Most premium parts have a lifetime replacement warranty! If it runs ok after it warms up then it aint plugs or wires. No way it SHOULD need those parts again at that mileage or time period. IF prem. parts were installed.
Does vehicle have a check engine light on? I assume vehicle is 4.6L- 5.4L has coil over plugs. If you didn't put too many miles on and is misfiring on particular cylinder suspect coil if 5.4L. Give us some more info so we can try and help- who's they and reason you took for service?