The motors of vacuum cleaners tend to run hot - exhaust air is used to cool them - an internal filter collects any dust that escapes the collecting bag else the dust will destroy the motor very quickly.
If the filter gets blocked the motor warms up and might catch fire. As a safety measure they usually fit an overheat cut-out switch. Sounds as if this is your problem.
I would guess that the internal filter needs attention.
Best not to run the cleaner without its internal filter.
Peccavi
November 2012