We have a 100 year old farm house that we bought 31 years ago. We have always had really hot water. The last few year in order to get hot water in the shower, I have had to turn the bath tub faucet on cold to hot until there is a clunking sound and I get the hot water. Then I can get hot water in the shower. If I don't do this the water is just warm. What is causing this?