I'm having a problem with the hot water in my shower. I turn on the water about mid way between the hot and cold and within in a minute I need to Turn the water to hot and keep turning it to the hot side until it's all the way to hot, but within a minute or so afterwards it's cold. I have a Moen signal handle facet. This seems to be the only place in the house that this happens but it is also the only shower in the house. Is there a problem with the facet or with the hot water system? I have oil heat. Help!