Boiler keeps going on & off?

ALPHA 280E - COMBI BOLER
When my CH & or DHW is on the boiler fires then when reaches temperature stops - it then fires again and during that time the warning light & overheat thermostat light come on, gas goes out then lights go out then boiler fires again - continues like this at intervals while boiler is on. Recently had a new pump & Overheat thermostat fitted. I think that the system radiators are hotter than normal