2007 Ford F150 engine light comes on

on a trip to texas engine light came on stopped at a dealer forund misfire on cylinder #2, #6 so the replaced the coilpack. We left drove 150 miles light came back on, kept driving when going up a mountain or hugh hill sometimes would misfire for a few seconds then straighten out, however light stayed on. Light went out after setting a few days left Texas going to Arizona again light came on same reasons. After getting back went to my dealer he found same reason on computer however say's it's not hte coil, but cannot find the problem.

We only haad 16,800 actual miles when this firt happened.