Scotty, I have a 2010 DTS with 17,000 actual miles.  Only drive it once a mouth to keep the battery up.  4.6 Northstar engine.  Starting getting random misfire code for cyl. 2.  Replaced plug ran fine for 20 miles or so and code came back.  Replaced coil pack and code went away.  20 miles or so later started getting random misfires on both cyl. 6 and 8.  Replaced plugs and trouble went away.  Now the car is running rough again after less than 30 miles.  Haven't read the code yet, but obviously I'm missing something.  This seems to start at idle and gets worse the longer you drive at all speeds.  I don't understand why the codes seems to move to other cyl.  Does the fact that everything so far is in the front bank of cyl. mean anything?