Fuel preasure should not be an issue unless its really starving for fuel, it would likly stuter, stumble and miss if it were running out of fuel preasure ,sounds like a programed derate.these are the things to check in order( sounds like they have been checked BUT ya never know! check 1 - Fault codes (Repair as nessary) check 2 - fuel preasure 15 to 20 psi under load. check 3 - re-pin fuel pump plug. if you have all of the above checked and done heres another place to go. i ran into this awhile back on an RD!( hicups and hesitations are USUALLY it is the pin conections at the fuel pump!)BUT Having said that ,You now get 1/2 the boost the engine should make ! You likly have a second unrelated problem If you have no faults coming up( No blink codes) , I would check to make sure the wiring to the torque limit switch on the trans is not shorted !The computer will not see a problem with this because it doesnt know theres a problem!(All it sees is a closed switch), and derates the power by half. check it out and see if that may lead you to a fix.