Jeff, So do you mean you can get hot out of it for two minutes, then it drops to luke warm and stays there for everafter? Does not get hotter or colder if you let it run for 10 for 20 minutes longer? Is this only at shower? Hot at other fixtures in this bath stay hot when turned on? You could quickly check for a cross connection by turning off the hot water tank. If you still get pressure on the valves hot side then you have the hot and cold tied together somewhere or a bad cartridge. The Fairfax is a pressure balanced valve, so if you shutoff the hot to the valve then you should get nothing from the hot or cold when you turn it on. If you do, then check for pressure on the hot side of a sink, if you have water then you have a cross connection. If no hot at sink then probably a bad cartridge.