I have a power vented gas model hot water heater that is about 5 years old. About every three or four days the main burner won't stay lit. The flame is even all around the burner with normal color. I will swithch it off then on and can see the ignitor heat up and the burner will light right away, but only stay lit for a few seconds. I will have to do this a few times until it stays lit and repeat this process in a few days. Would the sensor be the problem?