1993 Volvo 940 Knock Sensor?

Engine Mechanical problem
1993 Volvo 940 6 cyl Front Wheel Drive Automatic

My Volvo has been dying often and with no warning. I took it to a mechanic and he changed the knock sensor, but a few months later it starting dying again. Some says a few times then starts up again after sitting, and then I can drive a few days with no problems.