I rent a small garage apartment. When I turn on the hot water, sometimes it's hot -- but most of the time it is usually just warm enough to take a shower. Most of the time, by the end of the shower, the water turns cold (and I take quick showers!). I can turn the water on 30 minutes later, and it will be extremely hot. An hour later, it will be just warm again. There is no rhyme or reason to it. I know the hot water heater has been replaced, but this problem has happened both before and after the heater was replaced. Any ideas?
My first guess would be the anti scald valve inside the shower valve as some trash in it causing it to stick.
Have you checked the other faucets to check there temperture when that shower is messing up to see if there hot.