help can anyone fhelp me fix europa 24 combi boiler?

ive had boiler in house now for 4 years.up till now no problems now the boiler keeps firing up as if someone is running the water. it does this continuously. if we take a shower the water will be hot then cold as the boiler is switching on and off. we have heating but the same thing happens. the radiators stay hot as they dont get chance to go cold please!!!! can anybody help