I have a 50 Gal Kennmore water heater which is about 4 years old. I came home and my wife told me that there was no hot water. I lit the water heater and the water was fine for a day and now it has gone out again. It has a spark ignition and a window to see if it is lit. I hold the button down for 1 minute and then I push the ignition. It sparks and it is lit. I continue to hold for one more minute and turn it to the on position. I dont know if is working this time or not until I get home from work. Why would it keep going out?