I had this problem intermittently. I took it back to the dealer and they thought it was a problem with the engine management system. They changed a sensor and the ignition coil. Parts and labour cost £370.17. When I got it back the fault still occurred. They had to open up the cylinder and it seems that an exhaust valve had burnt out. It seems that the sensor was faulty and the engine managment system had shut down one of the cylinders causing the valve to burn out. I am facing another bill of £1500 to fix it.
My car is also 55 407 SW and we have had a long history of faulty sensors on the car (brakes, sunroof, etc). Fortunatley these had been fixed under warranty.