In our downstairs shower, we only have hot water for about 30 seconds and then it gets cold. After the 30 seconds, it's what I would call "cool" but it gets colder very quickly until it feels like ice water. Then if we turn off the water and wait a little bit and turn it back on we get 30 seconds of hot water again. There doesn't seem to be any difference no matter where the gauge is at (it in one of those showers where you have one knob that goes in a circle from cold to hot). Everywhere else in the house has normal hot water, including the sink in the same bathroom. And there is no problem with the water pressure. The stem was replaced fairly recently but nothing changed. What else could it be?