On trucks with A/C power control of the hydraulics, it's important to set both hydraulic pressures and amperage current limit using an a/c amp meter AND a pressure gauge. (Does this truck have a corrected [to show the clamp] data/capacity plate?, as required.)
Although I usually think that a 3 phase A/C motor in a forklift that has "chattering", especially at start-up of the motor, is more often than not the "encoder bearing", this may be as simple as adjusting the switch/sensor to match the pump ramp up with the valve's spool position.