I Have a 1998 Ingersoll Rand VR90B i have had the injection pump rebuilt all the filters changed fairly recently aswell. I also have a 12v lift pump. Currently has 11k hours The issue I'm having is at low rpms and in gear steering and braking will sometimes stall the engine it will put a good load on it and sometimes have to stop turning or let off the brake so it wont stall. The fork tilt will also put a good load on it aswell. It will climb hills ok but have to have it floored. It runs pretty well other than the issue with the hydraulics at low rpms
You need to check all your relief valves are not set putting a too much pressure in system loading up the pump. There is a procedure prob in service manual to set pressures. I had the same problem with my lull 844C engine was blowing smoke was under heavy load just after starting it not using any functions. My Gresen valve was bad worn replaced it no more under heavy load.