Hi Lawrie, this is almost definately the motor brushes being worn down. Relace them without delay because if the brushes wear so far down that the brush retainer comes in contact with the commutator (segmented part on motor that brushes come in contact with) this will without doubt render the motor defunct.
Also while you have brushes out, polish the segments with glass paper, NOT emery paper (even fine emery) as this is too course and will groove the copper segments and cause burn marks which causes bad contact from commutator to the brushes.
Hope this rectifies your problem