the most likely answer is that your motor brushes are worn, this fault will cause that model of machine to do what you have described. the brushes can be checked by removing them, (2 screws each) from underneath with the motor in situ with the machine unplugged from the mains of course, though its probably just as easy to take the motor off, obviously this could also be caused by a broken wire from the motor to the control module so check this also.
knowitall, May 2009