DewHot - NEVER constant

After a year of maing excuses about why the Dewhot "Constant Temperature Gas Geyser" delivers anything BUT constant temperature, I am now out of patience. It was installed properly, I made sure to have a pressure relase valve so pressure is not too high, I installed a water pump to make sure pressure is not too low (perhaps that is why it is never a constant temp), I have tried NOT to use a mixer, I have cleaned sieves, changed batteries. RIDICULOUS. So disappointed.