hi your problem you are having is most likely be motor related.inside the motor is a safety cut out device,it basically senses the heat from the motor.most often this thermal cut out becomes faulty and cuts in when the motor has only been running several minutes.failure of the carbon brushes,due to normal wear and tear wouldn't give you this symptom.the vac would simply run at intermittent speeds,which you would be able to hear,or simply wouldn't run.you will have to replace the entire motor :(
mech monster
November 2011