if the truck is running hot it will go into slow mode to try and assist in itself cooling down.
check the radiator and make sure it is not clogged up inside and outside. if this unit has hydraulic controlled fan unit make sure it is operating correctly. i have seen these hydraulic fan motors fail causing overheating. the temperature sensor on the fan motor may be bad or not getting power to turn the fan on.

just some thoughts i am having on this
good luck