repeat shutdown?

I have a EuroCombi A/23. The problems began with the temperature control not regulating until the boiler reaches maximum temperature. Once this happens the boiler keeps cutting in and out however the temperature remains on maximum. Also recently when I put the boiler on the overheat red light comes on and I have to turn on and off a few times before burner cuts in and starts working. I have had several plumbers to look at boiler and they all offer different reasons for faults. Please Advise