Central heating is freezing?

My Baxi 105e combi boiler is playing up.The hot water is great but the ch which is normally very good and warm is not heating the radiators at all. The timer is set and the temp is set at max. The boiler comes on at the desired time and the lights illuminate up to 80 degrees. Within 10 seconds of reaching 80, the burner light goes off and the lights start to go out down to 40 degrees. After 3 minutes the burner light is back on and temp is upto 80 again and the cycle continues. There is also a moaning noise from the boiler that sounds a little like an aeroplane engine slowing down. Any advice or diagnosis is much ppreciated.