This water heater is about 6 years old. I noticed the hot water supply not lasting long during a shower, or back to back showers. On the thermostat, I noted 3 lights lit. Going to the user manuel, the three lights indicated "System in lockout" If I turn off power to the unit and back on, the vent kicks on and tries to ignite the burner. It resets three times, and if the burner does not ignite by this time, it goes back into lockout. However, I've noticed that sometimes after taking a shower, the unit has kicked on igniting the burner and the lights go off. Then I'll notice the next morning it is in lockout again. Any ideas on what the problem is?