2001 f150 5.4l v8-have had issues running. Truck started idling rough and flashing check engine light with p303 code so we replaced the spark plugs and it ran ok for awhile. Few days ago I drove it about an hour and then it started doing this again once it was warmed up. Rough idle especially on acceleration, flashing check engine light, and code popping up again. Went through and double checked spark plugs and started it up and now it runs rough warmed up or not and runs worse when put into gear and has engine light stuck on. rpms also abnormally fluctuate and are lower than usual. What could be wrong?...

 

Could be a coil.