Bosch Aqua Star model 250SXO- natural gas installed since March 2007. The temperature is erratic causing the water which flows from the shower or tub to get hotter and hotter and then the unit shuts off and there is no hot water. This occurs on a consistent basis. Temperature has been re-set at the unit on several occasions without changing the erratic nature of operation. There is no help locally--I purchased this unit on an internet website. Considering the considerable cost of its purchase, perhaps there is some resource available. Any tips or suggestions would be greatly appreciated.