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.
~Critter