I have a duplex that is 10-years old and has the original hot water heater. Lately, I've been noticing that the temperature of the water is very erratic. It starts off hot but in a few minutes goes to luke-warm and I have to keep turning it hotter and hotter until I'm out of hot water (about 10 min). It never goes completely cold but i can consistently seem to keep a temperature while showering. Some days I notice this more than others, so it's not even a consistent problem. Do you have any advice as to why this may be happening or what I can do?