I seem to have a problem with the cylinder #3 misfireing. I change the plug and it is fine about a month later it goes bad again. This is weird the plug doesnt look fouled or anything. Every month this happens just on #3 any ideas what could be causing this. Using motorcraft platinum plugs to. Please help

 

I thought that was my problem once. Swapped all the plugs and it came back 2 weeks later. The coil was bad. Do you have coil on plugs?