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.
If this helps you out please accept the answer. Any more questions just ask.
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?