That truck should have an AC drive motor, and if so and you are unaware do not attempt messing with its internals, it could be a bad inverter, dust in the motors preventing ventilation, your work area ambient temperatures.

Fan shroud covered by foreign debris. If operators are Pushing loads with the truck instead of picking them up you will have massive issues.

They are called Fork Lifts not Fork Push for a reason. I need more info to help more...