Hello , I have a 97' F-150 V8 5.4 4X4. I changed the plugs about 2 months ago and it was running fine. It started acting up again (mis-firing). I went to auto zone and had it tested. They said the #5 cyclinder was the problem. I took out the plug and it was wore down pretty good. I replaced the plug but it still runs quite rough when accellerated.When I first start the truck it seems to run fine after a few minutes of driving it starts acting up. Should my next step be , Change the COP on the #5 cyclinder or is their a whole nother problem ? Thank You for your time. Forgot to add that I replaced the plugs with Motorcraft Platinum

 

Try swapping two of the COP's and see if the problem follows the COP.

 

Also make sure the COP is on the plug good. When I did plugs on my 01 I had a miss and the problems ended up being I didn't get the COP on all the way on the plug.