2001 Ford Mustang Running roughly at times

Car runs fine, then all of a sudden it starts to miss for about 10 to 15 seconds then it will start to run fine again. It doesn't do this all the time, just every now and then. It does this while driving, and while it sits still. We have installed a new battery, new spark plugs, plug wires, and coil pack. The check engine light was on at one time. We replaced some things as mentioned above. Check engine light reset. Has not came back on. However, the car still misses at times.