I have a '98 5.4l iter. A few years ago, I had the exact same symptons as you describe. I had just over 100k miles and thought it might be a spark plug going bad. After changing the plugs and checking everything else, couldn't figure it out. Never had a check engine light came on. I finally gave in and took it to the local Ford dealer. The problem was one bad oxygen sensor. Has been running fine every since.