There's a number of things it could be, but the most probable cause is a faulty fuel injector on that cylinder. Even though it is holding fuel pressure, it still may be bad.
When is it misfiring? Idle, accelerating under load, cruise, all the time, ???
Without special test equipment, there is no way I can tell you how to test it. You will have to have an injector balance test done on it to see if it is working properly.
You still could have other problems such as:
-no pulse (signal) to that injector from the PCM.
-vacuum leak somewhere near #5
-cracked spark plug
-possible worn distributor gear
-possibly worn camshaft lobe(s)
-or....there is a bulletin for the valve guides causing the valves to bind and stick. Typically, cylinders 3,4,5 or 6 will be the ones to experience this. Usually this happens when towing or on hard acceleration.
Make sure you don't have any wire damage at the injector harness/ connector. Unplug it and make sure there are no sign of fuel in there. You can also take the plastic cover off the top of connector and inspect the wiring for breaks. Sometimes the wires in the harness fatigue at the bend and pull inside the insulation.
Oh...and get rid of those Bosch plugs. Put some AC Delcos back in there.
Hope that helps somewhat.