Hello, how much slow speed you have? is it 1120 series?
If you have high mast, there is 8-meter switch which slow down the speed if mast is lifted more than that height. it doesn't show on display. there could be wire or sensor faulty. prior models had limit switch, and new ones are with proximity sensors.

if it moves very very slow less than 1 kph then it is RPM sensor on drive motor, wires also get too hot also.