My wife's car suddenly started running extremely rough yesterday morning, so today I got to play mechanic. When I popped the hood I immediately saw the problem. During the night we had either a Rat or a Squirrel get up under the hood and the critter went to work chewing a spark plug wire in half!
So, I ended up replacing the plug wires and while I was at it I changed the spark plugs.
Here is a photo of the plug wire:
notice how part of the wire is Completely Gone and the rubber boot has been partially chewed away!