HELP!!! Please.....99 Ford F-150
 

I have a 99 Ford F-150 4.6L V8 that will not run. The engine keeps misfiring when I am driving it. It idles fine but when given gas it stalls and misses. The code the computer is throwing is p320. I have replaced the fuel filter, spark plugs, crank shaft sensor and pig tail, cam shaft sensor. The wires have less than 5000 miles on them and seem in good working order according to the spark strength to the coils. I replaced the alternator and it ran OK for about a mile and then it started missing again.....WTF. I am at a loss as to what to try next other than replacing the coils but that is an expense that I would like to avoid if it could be something more simple. Any help would be appreciated.