Why is the arrow at the lower screen moving by itself?

Either your D-pad or digitizer is malfunctioning. The greater chances are that it is the D-pad and that something sticky has been spilled on it.You will need to dissassemble it to clean it. Instructions for doing that are here. Clean the D-pad button, the framing for it, the rubber pad, contacts on the power board and ribbon cable between the power board and motherboard along with the connectors with rubbing alcohol. Make sure the rubber pad moves freely and has no parts sticking out or in if there are any replace it. Inspect the cable between the power board and motherboard for breaks or small black lines if you find any replace it. If this doesn't work replace your power board. If that doesn't work replace your digitizer.