Hello Carolyn,
You say you changed the belt and pulley, did you notice the metal ferrul inside the pulley? they are prone to wear down and the favorite way to cure it is to take the metal ferrul out and the put it back in with a small piece of paper wedged between the ferrul and the pully, the pulley should be fairly tight to push on, the belt should be tight with only the minimum of slack, if you have done all that and it is still slow then WD40 is the answer on the machine, if that failks then it may be a motor problem.
Ken
drivewithken
May 2010