Hi Micheal,
Depends on the make and model my friend, usually for most makes it is overheating of the magnetron because either the air intake at the rear is blocked, or the fan isn't running.
Panasonic inverter models have 2 common shut offs apart from the above, 2 to 9 seconds, and 29 seconds. Sharp R9** series 59 seconds.
Contact me via microwave-repairs.co.uk if you need further help.
patrick, October 2010
