Marklift CH60C drive problems

Hello all, Im new here and am having an issue with my machine. Its a 1996 Marklift CH60C serial number 7390 4 wheel drive. The machine is in decient condition and works fairly well except for when you engage the drive function. When you engage the drive (either high or low speed) the engine bogs down and will stall like there is something restricting flow or something hydraulic bypassing somewhere. I have already gone through the governor system and it is functioning properly with the actuator responding as it should for the different inputs. Currently I have all 4 wheels off the ground and the engine wont stall the the hydraulics are still loading up to much on the engine. I do have the service manual for this machine, but unfortunately the hydraulic and electrical schematics are blurry and very hard to make out which circuit is which. I am well versed in electrical diagnostics and repair but I am not as good on hydraulics. If anyone more experienced than me would like to chime in and maybe lead me in a direction that i havent thought of, like where and how to check system and individual circuit pressures or what could be causing this issue, I would greatly appreciate it. Thank You