I have a problem with my New Holland 3630 tractor in the summer going up small hills it loses rpm and speed and when it cools down in the winter it hardly works.
I assume the fuel filters have been changed and the fuel strainer cleaned and checked. The electric fuel pump in the strainer is always a problem area on models that use it. Most of the problems with these is a lack of fuel, not leaving enough fuel for the injection pump. I would check this electric pump to see if it performs well. Remove the strainer line to the fuel filter and turn on to see what the flow is.