Hi,
- Can hear the electromotor running also accelerate and decelerate depending on the position of the joystick => the control system should be no problem but the speed of the pump motor seems to be reduced by unknown reason.
- When the truck is powered for half an hour it runs fine => this means the hydraulic system is OK.
=> I used to face a similar symptom in the past: Just open the control panel (display) to look for the battery 3VDC then replace it with the new one. This battery is responsible for feeding the power supply for the microcontroller of the display and it would be ready when the key switch ON. If it is over, when you turn ON the key SW the controller needs to wait for a while to warm up the controller, in your case is half of hour then it is ready for use.
Just try it and let me know the outcome
Good luck