Dip tube is a possibility. Another possibility is a moen single lever faucet in the house in another location. Every once in a while Moen Cartridges go bad. When they do, they allow water to pass through the faucet from the cold to the hot side (and visa versa), even when closed. So if you are using a shower, its drawing hot from the hot side and could be drawing cold from a tee connected to another single lever faucet (hot side) in the house some place else. Since your water system is basicly a hot system and a cold system separate, except where connected at faucets and the water heater, you can get crossover either at the dip tube on the water heater, or at single lever cartridge faucets anywhere in the house.