Installed Noritz model N-603-NG approximately 3 weeks ago. I got this unit used, it had been used about 2-3 years(worked fine they just didn't want to wait those extra seconds to get HW), it does not have the remote controller, it's installed outdoors, used a valve kit, have existing PRV on CW, previously heated HW with NG tank, no expansion tank. Problem is I lose HW pressure when using shower mixing valve (one handle for mixing water no volume control). I turn valve on all the way to full hot water position, wait for hw, then adjust valve handle down to mix in enough cold. At this point the pressure starts to diminish and then you lose the hw. Subsequently we've lost HW pressure to all fixtures in house. In the past we have regained HW pressure by turning off H & CW at valve kit, gas and electricity to heater and then turning them all back on. This remedy has stopped working as a temporary fix. Any ideas to resolve this problem would be greatly appreciated.