I'm having this same problem with 2 new valves in our Idaho summer home and also with 1 new and 1 older valve in our Calif home. You can get water temperature set just right and within 2 minutes it starts cooling down and you have to keep turning it higher and higher till its all the way to full hot water. I'm not running out of hot water nor is anyone else even using water while using the shower. Any information on this odd problem?