We had a 105 diesel JD at one time. When the combine got really warmed up the Hydro would act like it was slipping. The charge pressure would be low when this happened. In talking to an older farmer at the mill one day about the problem, he said to add a qt of 90W gear oil and if that didn't fix the problem to add a second qt. of 90 wt gear oil. I went home and added 2 qts.of the gear oil and it solved the problem. It ran that way till we sold the combine.