Obd2 codes I have a cylinder 5 misfire code P0305 ? How do I track this problem down?

I have replaced the spark plugs, Plug wires, Ignition coil pack, and the wiring harness wich was broken. I still get the same code as before which will not let me pass my emissions testing. The code is P0305. What is wrong? How do I track down this problem? On my 1998 Ford Econoline E150 van 6 cylinder version? I erased the code and it came back? Please HELP !!