As far as a replacement plug, I use Bosch Platinum 4304 in my 01 MGM, and they work great. If you've never changed the plugs on this type of car before, it is simple, but there are a few things to watch out for.
You will have just enough clearance to get the boot out from under the fuel rail once you've removed the hold down bolt on each coil. Be careful when you get to number 8, (near firewall on driver's side) because it does not have as much room to work on, and you will need to make sure that you have your wrench on the coil hold down bolt, so that you don't strip the head of bolt. (I replaced mine with Allen bolts).
Once you have plug lose (do not use too much force when breaking lose) then you can use a length of 3/8 fuel line to help get the plugs in and out of their holes. Stick the hose over the end of plug, and guide it back down into hole. Use anti-seize compound, and do not over torque the new plugs!
You may want to go out to the junk yard and practice on a Crown Vic / MGM that is sitting out there, if you have any doubts.
Also, if you have a misfire on one of your cylinders, try moving the coil/boot from one cylinder to another to see if misfire moves with the coil. More than likely the misfire is being caused by an injector. I would take a look at the wiring harness plug on the injector for number five as a suspect.