why does my upright bosch extraxx17 stop half way through cleaning 2 rooms?

My Bosch extraxx17 stops when I have cleaned 2 medium sized bedrooms I have cleaned all the filters completely,emptied the cylinder cleaned the brushes made sure all three screw fixes are in place when I put the cylinder dust tube back with a twist. It works perfectly then cuts out. It returns to normal after about an hour or when the motor cools. What should I do now? any clues? or should we look at the actual motor for dust etc:??

Lyn
November 2010