I have a Ruud Power Vent 2 hot water heater, model PVRP50-P. It was installed when the house was built, about 7 years ago. It is becoming more and more frequent that it stops heating the water and the water in the tank slowly cools down. If I unplug it and plug it back in it immediately starts back up. My guess would be that there is a problem with the thermostat. Does anybody have any suggestions?