We get frozen or burned when ever some one is in the shower and another person flushes a toilet or turns on the hot or cold water to a sink. If some one turns on the hot water, the shower gets cold. If someone flushes a toilet, the shower will nearly burn you. We have a pressure reduceing valve that has not be adjusted in 25 years but the water heater, a reliant, was replaced in about 1994 or 1995. I do not believe it is a dip tube problem, because there is lots of hot water if someone does not open any other water valves. I am surprised that even flushing a toilet will do this major temperature change to the shower. We notice this in both showers.