Have someone check the Long Term Fuel Trim. It should not be over 6-7% There are pretty common problems with the intake manifolds on these cars ( many are plastic ) I have chased misfires for this reason. Also, the Mass Air Flow Sensors get too lean and that too can cause P300s. ( random multiple misfire code ) Did you use the exact type of plug mentioned on the emissions label? You can't use after market plugs on these. they must be the special motorcraft platinums or they will never fire correctly. How did you test the plugs. Just because they look good doesn't mean they are firing up to spec. How many miles on the plugs? The 100,000 mile tune-up is a total myth. 60,000 max is more the reality. The 100,000 mile thing is a sales pitch, trust me.
Rather than just replacing parts, try to figure out what is wrong. Look at the fuel trim. It may not be the Mas Air Flow sensor.