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It could be a coil problem. Its a good idea to put quality wires on this engine. It is also important to use motor craft or autolite plugs. I've had other brands cause a rough running engine. The misfire moniter in the computer only sets a code after so many misfires. Its likely on yours that that engine hasn't missed enough to set a code. The diagnostic computer the ford dealer has can show which cylender has been missing befor the code sets and can watch a live graph of all the cylenders while the engine is running. They would be more likely to pinpoint your problem.
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Resembles a wet plug wire on an older vehicle. Not quite as severe as a completley dead cylinder. No backfiring.
Does it just do it at idle? Did you put Bosch spark plugs in it?