I have a gas hot water heater - Reliance with 40 gal tank. Was in the house when I bought it 4 years ago, so I'm not sure of its age. Recently the hot water will not flow for a minute or so, or comes out as just a trickle. Turn the faucet on and off a few times and it will flow again. Is there some kind of valve that would stick to prevent the flow? thx for the help