I'm having a problem with my hot water: sometimes it's not so hot. When I take a shower, I like the water to be quite hot, and half the time it is fine. But roughly half of my mornings, the water is not as hot as I would like. This is odd, since I am always the first one to use the shower or hot water each day, so it's not as if someone else has used up the hot water. When this happens, I usually turn the cold water all the way off, and the water still is only lukewarm. Could this be a thermostat problem on my hot water heater? It's a gas heater, and not more than 10 years old.