sounds like the clutch, if it as a round brush control knob on the left hand side as you look at it , Remove the brush bar clean any debris from the brissells and the two end caps make sure the bearing are spinning ok check for any obstructions, look down where the belt is if the white plastic pully is melted then you need a replacement clutch assy,
clutches go if you do not clean the brush roll ends regularly.
and always wash your top filters regulary too
Mr Cyclone
February 2008