I have a gas water heater about 15 years old, I flush it about once a year. The water seemed too hot ( measured 135 F at a faucet) , it had not been that hot before. There are about 10 temperature marks on the control[ It was on #4 , I moved it to #2 . Good temp, then 110 F. Wife thought it too cool so I set to #3 ; temp now 120 F. That was a week ago , today it is definitely cool ( not measured but about 100 F). I turned it back to #4 ; the burner came on and the water is now warm. I never had the temperature drift in a water heater before , Can this happen? The heater is a generic ( "State") from plumbing supply chain , not a big box store. There is no "bumping" to indicate stuff in the tank bottom.