2007 Mercury Mountaineer Cylinder 5 Misfiring

Shakes or Wobbles problem
2007 Mercury Mountaineer 6 cyl Two Wheel Drive Automatic 71K miles

Do you know what could cause for my check engine light to come on, as well as Gas cap light? I went in to an autozone and purchased a new one after clearing the code on my OB II, for a loose cap or bad cap. After 3 days it came back, but not only that but cylinder 5 is misfiring. I tried switching the wires to distingish if it was the wire and/ or coil pack, but got the same code (cylinder 5 misfiring). I went back to autozone and purchased a coil pack, but still the same what else could it be?