I would look at your extended warranty to see if it is covered. Some will cover others will not. Some will charge a deductible others will not.
I dont see any recall or TSB so far for this problem.
Have you done any work on the engine at all?
When was the last time the leads were replaced?
Have they ever been replaced or are they the originals?
How many miles you getting on the plugs before the problems occur?
Is Ford replacing all plugs when the problem has been addressed or are theyjust replacing the one that goes bad?
There are a few more things, but I think that should cover it for now.