Engine missing randomly
 

I have two 2001 Ford Expeditions that started a random engine miss at about 130K miles sometimes it runs fine and then it will miss for a while I changed spark plugs and fuel filter to no avail.Could it be one of the coils or plug wires? If so what is the eassiest way to check the coils? Do you think it may be an injector problem? This happened to both Expys at about the same milage.