Halstead Ace High boiler central heating cuts out before thermostat tempera?

Hi, my Halstead Ace High boiler central heating cuts out before thermostat temperature. Hot water works fine and switches back to CH when turned off. CH thermistor has been changed but it still cuts out. I have to top up the system a lot lately because it is low when it’s cold and too high when it’s running before cutting out. All radiators heat up and there is no sign of circulation problem. Any advice will be well appreciated.