I'm thinking it's a fuel/ignition system issue myself.....what is your idle RPM.....Waht RPM is it when you put the pedal to the floor in neutral with park brake on.....when you put the pedal to the floor and pull back on the tilt lever and put it into tilt bypass what is your RPM drop to?? I have seen trucks do this and was jut a simple power valve adjustment on the IMPCO 100 air mixer...this truck more than likely has the Mitsubishi 4G64 engine with an impco model J and IMPCO 100 air mixer....sounds like it is running on the lean side and starving for fuel under load.....also how clean is the air filter.....need both fuel AND air to make a correct fuel mixture....