boiler keeps anticycling?

I have a Halstead Ace High combination boiler,the boiler keeps shutting on/off either in ch mode or dhw mode hence heating does not get very hot or when in dhw mode goes cold after couple of minutes,some days boiler fine then starts doing it again for a day or so,the problem is gradually getting worse over the last 2 weeks,have changed the thermistor on the ch side but no avail,any ideas please.