Most modern motors are fitted with some kind of thermal switch/fuse to prevent overheating. I would suggest this is your problem if it works for a while and then goes off. There is an underlying problem, either a blockage or perhaps the filters require cleaning? Possibly the motor is at the end of its life. Is the brushbar turning ok? Think simple, before you go taking bits off. Remember, there is a filter above the motor, beneath the cassette.
Professor Higgins. (not really. Just a nickname!!), August 2004