If while cooking, your microwave stops after 59 seconds or one minute, you probably have a bad damper switch, damper motor or damper.
See the helpful photo here http://207.234.249.73/mwd/damper.jpg.
In my professional opinion, people should not work on a microwave unless they are fully informed and aware of the deadly hazards and necessary precautions.
One can find such information and very specific in our safety and disassermbly text files at http://www.microwavedisplay.com
You can find UK part suppliers and authorized servicers via http://www.sharp.co.uk
William Miller † MicrowaveDisplay.com, March 2007