I have a 97 F250 with the 5.4L V8. Started getting a P0306 code. Changed plug, swapped COP with #5, pulled injectors and swapped #5 and #6. Still polling the same code for a #6 misfire. Compression? Rings? Valve? or am I going down the wrong path and should be checking O2 sensor or EGR, or MAF? Any suggestions? Seems to run fine at 50 but get some bucking at 35-45. Not my primary vehicle so it has not been a real priority but starting to put some time in to get this solved.

 

How many miles are on the rest of the plugs? What brand plugs are you using? Are you identifying #6 correctly - 2nd plug back from the front on the driver's side?

A compression test wouldn't hurt.