Hi,
This is down to the thermal cutout on the motor. You may notice a smell before it happens. The thermal cutout gets warm and cuts the power to the motor to prevent it catching fire. After the thermal switch cools down (about 8 mins normally) the switch resets itself and will restart. Chances are the switch / motor may be faulty or is being overloaded. If its not being overloaded then its a faulty thermal switch and the whole motor will have to be replaced (its intergrated into the windings). I used to work in the QA department for the company that makes these mixers. They are called Belle and are located near buxton derbyshire. If you cant get anywhere with where you bought it from give them a call. 01298 84606.
Ex Belle Employee, March 2011