My shower has a single lever that turns from right (cold) to left (hot). When the lower bath faucet runs the water comes out hot. But the upper showerhead never heats up. If I run the lower faucet until the water is hot and then pull the tab so water comes through the showerhead, it comes out hot. But after a few minutes starts turning cold again! Im at my wits end! what could cause this and how do I fix it?