IF the problem gets worse as motor load increases the odds favor an ignition (spark) issue. IMO it's short sighted to look at a fuel problem until any/all ignition problems have been ruled out.
The 4.6 is terrible about collecting moisture inside the spark plug wells. Once the moisture gets in there it has no place to go. Have you inspected the spark plug wells, wires, and spark plugs?
Besides. IF this is a fuel related issue this would be easy to test for. IE the fuel pressure would drop as the problem started to occur.
I agree. On FI cars, the fuel pressure is constant as long as the pump is in order. And that you would find across the rpm range. If it were a carb'd car, different story. My truck had a sputter/bucking issue due to fuel boiling in the fuel line.
Check spark and alternator output at speed. If the coil is not getting enough voltage at rpm, it might also lead to an ignition problem.