Hot water goes on and off

I've got a rinnai tankless hot water heater that was installed in July '06 with a wall mounted thermostat. Just recently, I've noticed that sometimes the display on the thermostat is blank as if it is turned off. At other times it is on. Then when you turn on the hot water anywhere in the house, the display will sometimes go off and come back on and stay on which will provide you with hot water. At other times, the display will flash on and off. Of course when it is off, you will not have any hot water. Sometimes it will flash on and off, then go off for about 20 seconds, and then come on and stay on. Pushing the on/off button or the priority button on the thermostat does absolutely nothing. I have checked the plug in the attic and it is okay. Any suggestions?