This usually happens when the compressor is getting too hot. There's a device which is sensing the temperature of the compressor (for safety reasons). If the compressor gets too hot, it turns the compressor off to allow it to cool off. Yuo'll probably find that if you touch the compreesor (the big black pot at the back/bottom) it'll be too hot to touch for long. This is usually due to low refrigerant or a weak compressor.
PJC
January 2006