RPM variation Motor John Deere Retroexcavadora

I have the following problem: The Retro Jhon Deere 310 D has rpm variation in vacuum once the engine temperature is at normal regime and varies between 1500 and 2500 rpm being its 650-2500 rpm regime, that is, the release is raised in 1500 without accelerating. Check pump in bank and this correct one, I transferred it to another machine and also works correct. What could be happening so that I will accelerate in empty? Will the Torque converter? What other element could encourage us to accelerate alone? I hope someone gives me some clue. Victor