Hi andy.It could be that the brushes on your motor are worn.well worth taking back off machine and locating the brush holders.you might need star type allen key to undo them.When they are out you are likely to find one is more worn then the other,meaning it is not making contact with the commutator.If this is the case,simply buy new brushes.Mine cost me £20.00 Hopefully this will resole your problem.
john riley, February 2012