1st do what 'meliftman' suggests, then check/replace the return line filter and make sure the oil is the proper oil.
when you say, "use any working hydraulics" do you mean work the hydraulics at all (lifting an empty pallet 2 feet), or until they stall/load up the motor?
check the timing?

wouldn't hurt to check the hydraulic pressure, I have seen these problems occur when the hydraulic pressure is greater than it should be.

On most nissan motors running gas or LP they have a dash pot that is a vacuum actuated solinoid, that is located on the side of the carb or mixer right above the throttle shaft. When the motor is under a load the vacuum drops and the dash pot will extend and should increase the idle rpm to 1300-1800. If the Tia lift does not have one you should make sure idle is at about 7-750, If it is pnuematic make sure the tire pressure is at max pressure for steering. Make sure the Timing and fuel mixture are set right for your altitude. If it is Electronic fuel injected you will need to go in with a scanner and check the paramaters for the fuel system

I had the same problems same time behind, and is was solved simply through the adjustment of LPG Regulator.

i'll check it out, thanks

We have experienced the same problem on a Toyota. We corrected it by installing an "idle up" device which is standard equipment on some TCM's with Nisaan engines. When the engine is "taxed" when using hydraulics or steering, it automatically increses the engine rpm's.