Check the coil over plugs as well as the spark plugs. That was my problem
2001 Mustang GT - Crashed (Other person's fault) Stock.
2003 Mustang GT - Sold
Lowered, Flowmaster catbacks, CAI + Tune, 5% tint all around, American racing wheels.
2005 Mustang GT - Traded in
"Performance" spark plugs and ignition coils, 5% tint on back 3 - 20% on front two, 20 inch silver AMR wheels
2011 Mustang GT - BEAST
5% tint all around. 2014 GT premium wheels. Roush axleback