My Mastertemp 400 is about 3.5 years old and I had my first issue with it two days ago.
Starting temperature was 76 degrees (can't believe the water is still so cool in late May in Houston) and the target temp was set to 95. For the first 30-40 minutes the burner cycled on and off every 3-4 seconds and the temp slowly rose to the low 80's. The heater light flashed when it turned off meaning that it's waiting for the burner to ignite. The service light never came on. I've never had an issue with water flow or gas flow. I cleaned my cartridge filters over the weekend, but I don't believe that would cause a problem with water flow--it should improve it.
After about 40 minutes the unit finally stayed on and it reached the target temp after about 1 hour and twenty minutes. We had heavy rain throughout the day and I wonder if that had something to do with it. I haven't tried it again to see if it was the rain.
I've found a couple of threads with similar issues. Any ideas?