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Could be a coil pack failure. These failures are common at 50K+ on the V8 SHO engines, and somewhat less common, but still a problem, on the Duratecs with the same "coil-on-plug" design. They fail from excessive heat, and they can be very difficult to diagnose at first. One of the few tests that will help you out is a "load balance test" that can usually be performed only at the dealership. They will basically find out how much power each cylinder is putting out. The weak one(s) is/are the one(s) that you should try replacing the coil(s) on.

You could also remove each coil pack individually and test it in the following way:
-Inspect the coil and boot for cracks or other damage
-Shake the coil and listen/feel for rattling
-Test the resistance across the coil input terminals and compare to the other coils

It could also be common stuff... vacuum leaks, bad connections, etc. Check the IAB/IAC (Idle Air Bypass/Idle Air Control) which is right on top of the throttle body. Unplug the electrical connection and clean it out. I believe you can clean the IAB/IAC passage itself by removing the "accordion" hose on the intake and spraying TB/Carb cleaner through it while the engine idles.
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