I have a shower/bathtub setup where you pull a lever to divert the water from the bathtub faucet to the showerhead. Lately I have been having a problem maintaining hot water from the showerhead, but not from the bathtub faucet. Hot water comes out of the showerhead for anywhere from 3 to 5 minutes and then turns ice cold. When I divert the water back to the bathtub faucet it takes just a few seconds to get hot again, but if I leave it on the shower setting it remains cold. Once it's hot through the bathtub faucet I can again divert the water to the shower and it's hot again, although it always comes out in spurts for a few seconds from the showerhead after I do the diversion. I have already replaced the faucet valve and this does not happen in my other shower.I have a tankless water heater, if this matters at all. Any thoughts?