baxi combi instant 105e?

inheating mode temp. goes to 50`c then turns off . i have changed both temp. sensers. burner stays on in DHW mode . sometimes burner is on but neon is not always lit .. diverter valve operates. when DHW selector knob turned on ,boiler goes to preheat mode then changes back to heating but burner only on for 10sec then off and will not come back on untill rest operated