I have a high efficiency water heater (Polaris PG Model) with an extra tank to store extra hot water in it. Our home has a continuous circulating hot water system to keep things instantly hot. A few weeks ago my circuit board went out and was replaced, since then every shower starts hot for 10 minutes, goes cold for a few minutes, then goes back hot again. I checked the circulating pump its working fine and is on the correct time, High Eff water heater doesn't have a dip tube so I know it wasn't that, burners are working fine and thermostat is working good. Any suggestions? I think it would be odd if all three shower valves need work at the same time. Thanks for your help.