I fixed my neighbours car with that fault,it was a dirty commutator in the motor,I recall stripping the motor in situ cleaning the commy with sand paper (it was very burnt) and putting it back together,it worked a treat.If you have ever changed brushes or did work on an electric motor you will know the score it is not difficult but you will need two pairs of hands (someone to hold the brushes back when you put the innards back in) and a little bit of ingenuity.Good luck.
Rich t
May 2015