I do not have nor have not worked on a 5.4, but the way I understand is that the 5.4 has a COP system or Coil Over Plug. So there is one coil for each plug. 8th cylinder should be drivers side rear cylinder.

It could be from a bad coil, sparkplug or a sticky injector. Most likely the plug or coil though. Remember, start with the cheap stuff first!