my hill billy terraine motorised trolley goes for short distances then stop?

when my motorised hill billy terrain is put into distance mode i.e 10 mtrs 20 etc when it completes cycle i have to disconnect then connect the battery to get it going again.It also goes for short distances when used with normal mode and i have to perform battery action again. I have renewed the potentiometer and still the problem persists.Any clues ? Thanks.